build: Avoid building files multiple times

We don't have to build each source for every test, instead we could
create a static library link it with every test, which will be a lot
faster.

plugins/dummy/meson.build

@@ -51,6 +51,7 @@ calls_dummy = shared_module(
   dummy_sources,
   dependencies: dummy_deps,
   include_directories: src_include,
+  link_with: libcalls,
   install: true,
   install_dir: dummy_install_dir
 )