Makefile+action: add make unit-module to test submodules

This commit adds a new command `make unit-module` to run unit tests for
submodules to avoid future build errors.

.github/workflows/main.yml

@@ -186,6 +186,8 @@ jobs:
           - unit tags="kvdb_postgres"
           - unit tags="kvdb_sqlite"
           - btcd unit-race
+          - unit-module
+
     steps:
       - name: git checkout
         uses: actions/checkout@v3
@@ -217,6 +219,7 @@ jobs:
           path-to-profile: coverage.txt
           parallel: true
 
+
   ########################
   # run ubuntu integration tests
   ########################