Add automated tests that run a GDExtension (rather than just building it)

test/run-tests.sh

@@ -0,0 +1,24 @@
+#!/bin/bash
+
+GODOT=${GODOT:-godot}
+
+END_STRING="==== TESTS FINISHED ===="
+FAILURE_STRING="******** FAILED ********"
+
+OUTPUT=$($GODOT --path project --debug --headless --quit)
+ERRCODE=$?
+
+echo "$OUTPUT"
+echo
+
+if ! echo "$OUTPUT" | grep -e "$END_STRING" >/dev/null; then
+    echo "ERROR: Tests failed to complete"
+    exit 1
+fi
+
+if echo "$OUTPUT" | grep -e "$FAILURE_STRING" >/dev/null; then
+    exit 1
+fi
+
+# Success!
+exit 0