Run internal tests in Cygwin (#1592)

test/run-tests.sh

@@ -6,13 +6,15 @@ cd "$(dirname "$0")"
 usage() {
 	echo "Runs regression tests on RGBDS."
 	echo "Options:"
-	echo "    -h, --help      show this help message"
-	echo "    --only-free     skip tests that build nonfree codebases"
+	echo "    -h, --help          show this help message"
+	echo "    --only-free         skip tests that build nonfree codebases"
+	echo "    --only-internal     skip tests that build external codebases"
 }
 
 # Parse options in pure Bash because macOS `getopt` is stuck
 # in what util-linux `getopt` calls `GETOPT_COMPATIBLE` mode
 nonfree=true
+external=true
 FETCH_TEST_DEPS="fetch-test-deps.sh"
 while [[ $# -gt 0 ]]; do
 	case "$1" in
@@ -24,6 +26,9 @@ while [[ $# -gt 0 ]]; do
 			nonfree=false
 			FETCH_TEST_DEPS="fetch-test-deps.sh --only-free"
 			;;
+		--only-internal)
+			external=false
+			;;
 		--)
 			break
 			;;
@@ -49,6 +54,10 @@ for dir in asm link fix gfx; do
 	popd
 done
 
+if ! "$external"; then
+	exit
+fi
+
 # Test some significant external projects that use RGBDS
 # When adding new ones, don't forget to add them to the .gitignore!
 # When updating subprojects, change the commit being checked out, and set the `shallow-since`