meson: ci: ensure tests are built before running them

 meson: ci: ensure tests are built before running them
 
 Meson 1.7 stopped building all the dependencies of tests as part of the
 default build target. But it does breaks CI because we only built the default
 target before running the test, and ran the tests with --no-rebuild.
 
 The simplest fix would be to remove --no-rebuild from MTEST_ARGS, but it seems
 better to explicitly build the test dependencies, so compiler warnings /
 errors are visible as part of the build step.
 
 Discussion: https://postgr.es/m/CAGECzQSvM3iSDmjF+=Kof5an6jN8UbkP_4cKKT9w6GZavmb5yQ@mail.gmail.com
 Backpatch: 16-, where meson was added