changing tests/qtest/meson.build causes unnecessary rebuilding

[Peter Maydell]

$ make -C build/arm-clang/ -j8
make: Entering directory
'/home/petmay01/linaro/qemu-from-laptop/qemu/build/arm-clang'
[1/23] Generating qemu-version.h with a meson_exe.py custom command
make: Leaving directory
'/home/petmay01/linaro/qemu-from-laptop/qemu/build/arm-clang'
$ touch tests/qtest/meson.build
$ make -C build/arm-clang/ -j8
make: Entering directory
'/home/petmay01/linaro/qemu-from-laptop/qemu/build/arm-clang'
/usr/bin/ninja  build.ninja && touch build.ninja.stamp
[0/1] Regenerating build files.
The Meson build system
[...]

It then goes on to rebuild hundreds or thousands of files, most of
which are not even in tests/. (Oddly, the exact set of files recompiled
seems to vary from run to run.)

Forcing a full rebuild of all of QEMU seems rather excessive when
the change was likely "add a new test case"...

thanks
-- PMM