run-tests: Split out building and running of tests into a Makefile

This allows building and running the tests in parallel.

.github/workflows/build.yml

@@ -258,6 +258,7 @@ jobs:
           msystem: mingw64
           install: >-
             unzip
+            make
       - uses: actions/download-artifact@v3
         with:
           name: windows-${{matrix.arch}}-toolchain