[42] tests: Give mutter-test-runner the RUNPATH to find Clutter

Until recently, mutter-test-runner called into libraries that indirectly depend on (mutter's fork of) Clutter, but did not actually call into Clutter itself. Commit 1bf70334 "tests/runner: Make test runner use the headless backend" gave it a direct call into Clutter, which means the runtime linker will fail unless the executable's RUNPATH is sufficient to find Clutter.

For future-proofing, do the same for the other test executables.

Resolves: #2389 (closed)

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This is the version I have actually tried (we don't have mutter 43 in Debian yet). Untested GNOME 43 version in !2581 (merged).