meson: Add target for installing test files & improve install_test_files

The changes in b6a0d469cae prevented installation of the test files during a
normal install. However, the buildfarm intentionally tries to trun the tests
against a "real" installation. The new install-test-files target provides that
ability.

Because we want to install into a normal directory, I removed the necessary
munging of the target paths from meson.build and moved it into
install-test-files. I also added DESTDIR support, so that installing can
redirect the directory if desired. That's used for the tmp_install/
installation now.

I didn't like the number of arguments necessary for install_test_files, so I
changed it to use
  --install target list of files
which makes it easier to use for further directories, if/when we need them.

Discussion: https://postgr.es/m/20230308012940.edexipb3vqylcu6r@awork3.anarazel.de

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