CI: Run macOS tests as debug

These are dog-slow at building, and the tests themselves are barely
sped up running as release.

Given that we have ~10 minute build and ~3 minute test time on Github
Actions on macOS, let's see if this speeds it up

(we can also do it for the others, but the most important is the
slowest test because that's what stops the checkmark appearing)

.github/workflows/main.yml

@@ -163,7 +163,7 @@ jobs:
     - name: cmake
       run: |
         mkdir build && cd build
-        cmake -DWITH_GETTEXT=NO ..
+        cmake -DWITH_GETTEXT=NO -DCMAKE_BUILD_TYPE=Debug ..
     - name: make
       run: |
         make VERBOSE=1