docs: Update README for experimental testmode

[chibicode]

• Fix the npm command
• Make it clear where to place test files
  • Was reported here: Playwright could not find tests with Nextjs 15 experimental testmode  #71773
  • Relevant Next.js code:

next.js/packages/next/src/experimental/testmode/playwright/default-config.ts

Line 13 in ae5026a

testMatch: '{app,pages}/**/*.spec.{t,j}s',

• Make it clear that next.onFetch only works for external requests.
  • Relevant Next.js code:

next.js/packages/next/src/experimental/testmode/playwright/page-route.ts

Lines 34 to 40 in d2711d2

	// Continue the local requests. The followup requests will be intercepted
	// on the server.
	const pageOrigin = new URL(page.url()).origin
	const requestOrigin = new URL(request.url()).origin
	if (pageOrigin === requestOrigin) {
	return continueRoute(route, request, testHeaders)
	}

[ijjk]

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• approve CI run for commit: 37f7364

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[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	vercel/next.js chibicode/update-testmode-docs	Change
buildDuration	27s	27s	N/A
buildDurationCached	22.7s	19.1s	N/A
nodeModulesSize	392 MB	392 MB	N/A
nextStartRea..uration (ms)	544ms	718ms	⚠️ +174ms

Client Bundles (main, webpack)

	vercel/next.js canary	vercel/next.js chibicode/update-testmode-docs	Change
5306-HASH.js gzip	54.2 kB	54.2 kB	N/A
8276.HASH.js gzip	169 B	168 B	N/A
8377-HASH.js gzip	5.46 kB	5.46 kB	N/A
bccd1874-HASH.js gzip	52.9 kB	52.9 kB	N/A
framework-HASH.js gzip	57.5 kB	57.5 kB	N/A
main-app-HASH.js gzip	240 B	242 B	N/A
main-HASH.js gzip	34.5 kB	34.5 kB	N/A
webpack-HASH.js gzip	1.71 kB	1.71 kB	N/A
Overall change	0 B	0 B	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js chibicode/update-testmode-docs	Change
polyfills-HASH.js gzip	39.4 kB	39.4 kB	✓
Overall change	39.4 kB	39.4 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js chibicode/update-testmode-docs	Change
_app-HASH.js gzip	193 B	193 B	✓
_error-HASH.js gzip	193 B	193 B	✓
amp-HASH.js gzip	512 B	510 B	N/A
css-HASH.js gzip	343 B	342 B	N/A
dynamic-HASH.js gzip	1.84 kB	1.84 kB	✓
edge-ssr-HASH.js gzip	265 B	265 B	✓
head-HASH.js gzip	363 B	362 B	N/A
hooks-HASH.js gzip	393 B	392 B	N/A
image-HASH.js gzip	4.59 kB	4.58 kB	N/A
index-HASH.js gzip	268 B	268 B	✓
link-HASH.js gzip	2.35 kB	2.35 kB	N/A
routerDirect..HASH.js gzip	328 B	328 B	✓
script-HASH.js gzip	397 B	397 B	✓
withRouter-HASH.js gzip	323 B	326 B	N/A
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Overall change	3.59 kB	3.59 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js chibicode/update-testmode-docs	Change
_buildManifest.js gzip	748 B	747 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js chibicode/update-testmode-docs	Change
index.html gzip	524 B	523 B	N/A
link.html gzip	539 B	538 B	N/A
withRouter.html gzip	520 B	520 B	✓
Overall change	520 B	520 B	✓

Edge SSR bundle Size

	vercel/next.js canary	vercel/next.js chibicode/update-testmode-docs	Change
edge-ssr.js gzip	130 kB	130 kB	N/A
page.js gzip	211 kB	210 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	vercel/next.js chibicode/update-testmode-docs	Change
middleware-b..fest.js gzip	674 B	673 B	N/A
middleware-r..fest.js gzip	155 B	156 B	N/A
middleware.js gzip	31.3 kB	31.3 kB	N/A
edge-runtime..pack.js gzip	844 B	844 B	✓
Overall change	844 B	844 B	✓

Next Runtimes

	vercel/next.js canary	vercel/next.js chibicode/update-testmode-docs	Change
app-page-exp...dev.js gzip	393 kB	393 kB	N/A
app-page-exp..prod.js gzip	132 kB	132 kB	✓
app-page-tur..prod.js gzip	145 kB	145 kB	✓
app-page-tur..prod.js gzip	141 kB	141 kB	✓
app-page.run...dev.js gzip	381 kB	380 kB	N/A
app-page.run..prod.js gzip	128 kB	128 kB	✓
app-route-ex...dev.js gzip	39.3 kB	39.3 kB	✓
app-route-ex..prod.js gzip	25.6 kB	25.6 kB	✓
app-route-tu..prod.js gzip	25.6 kB	25.6 kB	✓
app-route-tu..prod.js gzip	25.4 kB	25.4 kB	✓
app-route.ru...dev.js gzip	40.9 kB	40.9 kB	✓
app-route.ru..prod.js gzip	25.4 kB	25.4 kB	✓
dist_client_...dev.js gzip	356 B	356 B	✓
dist_client_...dev.js gzip	349 B	349 B	✓
pages-api-tu..prod.js gzip	9.69 kB	9.69 kB	✓
pages-api.ru...dev.js gzip	11.8 kB	11.8 kB	✓
pages-api.ru..prod.js gzip	9.68 kB	9.68 kB	✓
pages-turbo...prod.js gzip	21.9 kB	21.9 kB	✓
pages.runtim...dev.js gzip	31.5 kB	31.5 kB	✓
pages.runtim..prod.js gzip	21.9 kB	21.9 kB	✓
server.runti..prod.js gzip	60.5 kB	60.5 kB	✓
Overall change	896 kB	896 kB	✓

build cache Overall increase ⚠️

	vercel/next.js canary	vercel/next.js chibicode/update-testmode-docs	Change
0.pack gzip	2.1 MB	2.11 MB	⚠️ +5.2 kB
index.pack gzip	74.9 kB	75.4 kB	⚠️ +461 B
Overall change	2.18 MB	2.18 MB	⚠️ +5.66 kB

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Commit: 37f7364