unit-tests: Add a TESTS_PLUGINS environment variable

This is often more convenient than specifying plugins in a configuration file.

src/libstrongswan/tests/test_runner.h

@@ -69,10 +69,14 @@ struct test_configuration_t {
  *
  * - TESTS_VERBOSITY: Numerical loglevel for debug log
  * - TESTS_STRONGSWAN_CONF: Specify a path to a custom strongswan.conf
+ * - TESTS_PLUGINS: Specify an explicit list of plugins to load
  * - TESTS_SUITES: Run specific test suites only
  * - TESTS_SUITES_EXCLUDE: Don't run specific test suites
  * - TESTS_REDUCED_KEYLENGTHS: Test minimal keylengths for public key tests only
  *
+ * Please note that TESTS_PLUGINS actually must be implemented by the init
+ * callback function, as plugin loading is delegated.
+ *
  * @param name			name of test runner
  * @param config		test suite constructors with dependencies
  * @param init_cb		init/deinit callback

src/libstrongswan/tests/tests.c

@@ -37,7 +37,8 @@ static bool test_runner_init(bool init)
 	{
 		char *plugins, *plugindir;
 
-		plugins = lib->settings->get_str(lib->settings,
+		plugins = getenv("TESTS_PLUGINS") ?:
+					lib->settings->get_str(lib->settings,
 										"tests.load", PLUGINS);
 		plugindir = lib->settings->get_str(lib->settings,
 										"tests.plugindir", PLUGINDIR);