GnuTests: Split online process to a script

[oech3]

Current CI:

1. Version is extracted by grep from script, but script is actually not a version specific. It is a confusion.
2. build-gnu.sh is existing for offline test, but actually has online ./bootstrap
3. Difficult to backport tests at offline.
4. Depending on git.

So detatched online part.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/overlay-headers (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/cp/cp-mv-enotsup-xattr is now passing!
Congrats! The gnu test tests/mkdir/writable-under-readonly is now passing!

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/overlay-headers (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/cp/cp-mv-enotsup-xattr is now passing!
Congrats! The gnu test tests/mkdir/writable-under-readonly is now passing!

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/overlay-headers (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/cp/cp-mv-enotsup-xattr is now passing!
Congrats! The gnu test tests/mkdir/writable-under-readonly is now passing!