tests: exercise the RUNTIME_PREFIX feature

Originally, we refrained from adding a regression test in 7b6c6496374
(system_path(): Add prefix computation at runtime if RUNTIME_PREFIX set,
2008-08-10), and in 226c0ddd0d6 (exec_cmd: RUNTIME_PREFIX on some POSIX
systems, 2018-04-10).

The reason was that it was deemed too tricky to test.

Turns out that it is not tricky to test at all: we simply create a
pseudo-root, copy the `git` executable into the `git/` subdirectory of
that pseudo-root, then copy a script into the `libexec/git-core/`
directory and expect that to be picked up.

As long as the trash directory is in a location where binaries can be
executed, this works.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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