Add GitHub Actions configuration file to enable CI

There are several builds defined here, and a few more sketched out that
don't quite work.

Most of the steps call out to external shell or batch files,
to make things easier to keep track of.

.github/workflows/scripts/mac/run-tests.sh

@@ -0,0 +1,7 @@
+#!/bin/sh -ve
+
+cd test
+
+# These are the same command-line arguments that are passed in the Xcode scheme.
+# If the scheme is changed, these should be updated too.
+../proj/xc12/build/Release/boe_test -i --order lex