tests: Use CALLS_PLUGIN_DIR to search for plugins in the build dir

There is no need to add a second variable to search for plugins, we can
use CALLS_PLUGIN_DIR directly.

plugins/provider/tests/meson.build

@@ -32,12 +32,12 @@ test_env = [
   'CALLS_AUDIOSRC=audiotestsrc',
   'CALLS_AUDIOSINK=fakesink',
   'GSETTINGS_SCHEMA_DIR=@0@/data'.format(meson.project_build_root()),
+  'CALLS_PLUGIN_DIR=@0@/plugins'.format(meson.project_build_root()),
 ]
 
 test_cflags = [
   '-DFOR_TESTING',
   '-Wno-error=deprecated-declarations',
-  '-DPLUGIN_BUILDDIR="@0@"'.format(full_calls_plugin_builddir),
 ]
 
 test_link_args = [