build: Define test_sources, don't append
If building without git support, meson setup fails with
src/plugins/meson/meson.build:30:0: ERROR: Unknown variable "test_sources".
I believe the += a mistake in commit c2e10cde ("meson: fixed language parsing in meson files") which wasn't noticed because there was an existing test_sources variable (in the git plugin's meson.build, see next commit).