internal/frontend/fetchserver: add test for "synctest" shortcut

The previous commit 5cd4436 added tests
that internal packages are not found by a shortcut. This change adds a
test that a package (like synctest) which has an internal and public
package is found.

Updates golang/go#76136

Change-Id: I80297ee6fff7fbb1c2d5bd2188a387a39f24ac35
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/718280
TryBot-Bypass: Nicholas Husin <[email protected]>
Reviewed-by: Nicholas Husin <[email protected]>
Reviewed-by: Junyang Shao <[email protected]>
Reviewed-by: Nicholas Husin <[email protected]>
Auto-Submit: Nicholas Husin <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: bystones

internal/frontend/fetchserver/server_test.go

@@ -169,6 +169,14 @@ var testModules = []testModule{
 				name:   "asan",
 				suffix: "internal/asan",
 			},
+			{
+				name:   "synctest",
+				suffix: "internal/synctest",
+			},
+			{
+				name:   "synctest",
+				suffix: "testing/synctest",
+			},
 		},
 	},
 	{
@@ -704,6 +712,12 @@ func serverTestCases() []serverTestCase {
 			urlPath:        "/boring",
 			wantStatusCode: http.StatusNotFound,
 		},
+		{
+			name:           "stdlib shortcut (testing/synctest) with internal package (internal/synctest)",
+			urlPath:        "/synctest",
+			wantStatusCode: http.StatusFound,
+			wantLocation:   "/testing/synctest",
+		},
 		{
 			name:           "package page with trailiing slash",
 			urlPath:        "/github.com/my/module/",