Refactor the test harness for external repo tests (#1994)

- Use CTest labels to filter tests ("internal"/"external", "free"/"nonfree",
  and individual tool+project names) instead of defining `TESTS_RUN_NONFREE`
- Allow each external test to run independently in CTest
- Remove the unused no-op `fetch-test-deps.sh --only-internal` option

test/external/test.sh

@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+export LC_ALL=C
+
+# Game Boy release date, 1989-04-21T12:34:56Z (for reproducible test results)
+export SOURCE_DATE_EPOCH=609165296
+
+cd "$(dirname "$0")/.."
+
+RGBDS_PATH="RGBDS=../../"
+
+if [ ! -f "external/$1.cfg" ]; then
+	echo >&2 'External test file '"$1"'.cfg does not exist'
+	exit 1
+fi
+
+# Sourcing "external/$1.cfg" defines `EXTERNAL_TEST_*` values used below.
+. "external/$1.cfg"
+
+if ! cd "$EXTERNAL_TEST_REPO"; then
+	echo >&2 'Please fetch test deps before running any external test'
+	exit 1
+fi
+make clean $RGBDS_PATH
+make -j4 "$EXTERNAL_TEST_TARGET" $RGBDS_PATH
+hash="$(sha1sum -b "$EXTERNAL_TEST_FILE" | head -c 40)"
+if [ "$hash" != "$EXTERNAL_TEST_HASH" ]; then
+	cat >&2 <<EOM
+error: "$EXTERNAL_TEST_FILE" checksum did not match!
+    Expected $EXTERNAL_TEST_HASH,
+         got $hash
+EOM
+	exit 1
+fi