refactor(tests): Use GH Actions matrix for tests.

* To parallelize our tests, generate a dynamic matrix
  of tests to run.

app/run-test.sh

@@ -15,7 +15,7 @@ fi
 
 testcases=$(find $path -name native_posix_64.keymap -exec dirname \{\} \;)
 num_cases=$(echo "$testcases" | wc -l)
-if [ $num_cases -gt 1 ]; then
+if [ $num_cases -gt 1 ] || [ "$testcases" != "$path" ]; then
 	echo "" > ./build/tests/pass-fail.log
 	echo "$testcases" | xargs -L 1 -P ${J:-4} ./run-test.sh
 	err=$?