Fixed "make check" for cmake build

The check target did not depend on the test_plugin binary, so this would
cause "make check" to fail at times because of the absence of
test_plugin. This change adds a dependency on test_plugin so that it
will always get built before the tests are executed.

cmake/tests.cmake

@@ -214,4 +214,5 @@ target_link_libraries(lite-arena-test libprotobuf-lite gmock_main)
 
 add_custom_target(check
   COMMAND tests
+  DEPENDS tests test_plugin
   WORKING_DIRECTORY ${protobuf_source_dir})