Update testing README

[daljit46]

Addresses #2826.
Testing tips and instructions have been updated to reflect the transition to CMake. Sections on the handling of testing data assume #2848 has been merged.
Some instructions may not be clear or need removing due to the fact that some of the concerns/workflows no longer apply with the transition to CMake.

It's still WIP.

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[daljit46]

I've updated the testing instructions to reflect the latest changes in testing code. @Lestropie could you please have a look and see if there's anything that's missing and/or incorrect?

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[Lestropie]

• There are now labels both for language and for each command name, so no need to use regexes.
• There were still some out-of-date references to being one test per line of a test script file, predating Testing: shell script per test #2865.
• The later text read like it was written when we first added the testing. I've replaced it with something new describing how I typically go about it these days.

[daljit46]

I think you've removed the section on using a local clone to download the test data. Shall we omit that in the testing readme? I think it'd be for that mechanism to be documented somewhere at least.

[Lestropie]

Good catch. Think I failed to move it partway through, then erroneously deleted it entirely when I had to do an explicit rebase.

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[daljit46]

This seems to be good to go IMO, so will merge soon.