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