mtest: Chdir into the build directory before running tests with -C

When `ninja -C builddir/ test` is run, ninja will change into the build
dir before starting, but `meson test -C builddir/` does not. This is
important because meson does not use (for good reasons) absolute paths,
which means if a test case needs to be passed as an argument a file name
that is part of the build process, it will be relative builddir. Without
changing into the builddir the path will not exist (or worse, point at
the wrong thing), and test will not behave as intended.

To fix this mtest will change directory before starting tests, and will
change back after all tests have been finished.

Fixes #2710

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