Base/CI: Create and check test-results.log file for on-target tests

Change run-tests-and-shutdown.sh to output a dead simple results file
that just records how many tests failed.

In the CI script, mount the _disk_image after running tests and verify
that the number of failed tests is 0. Otherwise, fail the build :^)

While we're here, bump the timeout for the tests up to 30 minutes, to
make sure that less powerful runners don't fail the job unecessarily.

Base/home/anon/tests/run-tests-and-shutdown.sh

@@ -47,6 +47,8 @@ if test $fail_count -gt 0 {
     echo "==== Failing tests: $failed_tests ===="
 }
 
+echo "Failed: $fail_count" > ./test-results.log
+
 if test $DO_SHUTDOWN_AFTER_TESTS {
     shutdown -n
 }