cmake: Pass the source directory to the tests

Our tests typically run in their own environment, which is great for
normal tests.

However for the coming translation test, we don't want to copy the .po
files into the test environment, so it's nice to have a way out.

cmake/Tests.cmake

@@ -38,6 +38,7 @@ if(POLICY CMP0037)
 endif()
 add_custom_target(test
   COMMAND env CTEST_PARALLEL_LEVEL=${CTEST_PARALLEL_LEVEL} FISH_FORCE_COLOR=1
+          FISH_SOURCE_DIR=${CMAKE_SOURCE_DIR}
           ${CMAKE_CTEST_COMMAND} --force-new-ctest-process # --verbose
           --output-on-failure --progress
   DEPENDS fish_tests tests_buildroot_target