Improve test suite isolation, plus miscellaneous test suite changes

[tomeon]

First, thanks for clipmenu -- it has upped my clipboard game significantly :)

While working on a different PR, I ran the clipmenu test suite and was surprised to find that it clobbered my clipboard cache. This PR is my attempt to isolate the test suite from the user's existing state; the PR also introduces some miscellaneous additional changes.

Summary of changes:

1. Introduce a test fixtures hierarchy. The fixtures directory contains a bin path with executables that replace the executables previously dynamically generated in tests/test-clipmenu and tests/test-perf.
2. Prepend the test/fixtures/bin directory and the clipmenu repository toplevel directory to PATH. This makes it possible to run (say) clipctl cache-dir and be confident that the selected clipctl executable is <toplevel>/clipctl.
3. Put all generated assets (including the clipboard cache file) into a shared temporary directory (tempdir=$(mktemp -d)). This simplifies end-of-test filesystem cleanup (just recursively remove tempdir).
4. Add a cleanup function that runs as a trap on the EXIT pseudo-signal. This function recursively removes the above-mentioned tempdir, and also provides a cleanup_pre hook for running code prior to removing the temporary directory hierarchy. tests/test-clipmenu uses cleanup_pre to print the contents of the temporary file used for capturing the mocked-up launchers' output.
5. Consolidated common test setup (including some of the things mentioned above) into the file tests/setup.sh, and source this file from tests/test-clipmenu and tests/test-perf.
6. In tests/test-perf, ensure that we always generate 1500 clipboard files. Previously, if NO_RECREATE was in effect and a previous test run generated fewer that 1500 clipboard files (say, the test run was interrupted during the generation stage), subsequent runs with NO_RECREATE in effect would not generate the remaining needed files.
7. Print a diagnostic message in the cleanup trap function when a test script failed. The diagnostic message includes the command that failed, to aid in debugging.
8. Use a dynamically-assigned BASH_XTRACEFD in the test/fixtures/bin/clipmenu-perf wrapper rather than using the default stderr file descriptor.
9. Define a PS4 that results in xtrace displaying additional execution context -- filename, line number, and (if applicable) shell function.

Thanks again for clipmenu, and thanks in advance for your consideration.

[cdown]

clipmenu is now in C, so closing since this applies to the old bash version. Thanks for your work on this, though!

(note: rofi-script support is not in clipmenu 7, if that's a problem, let's talk about it)