btest/plugin: Add plugin testing enum identifiers

This plugin has a generic name, but for now just tests the API around
enum types and enum vals at InitPostScript() time.

testing/btest/plugins/api-plugin.zeek

@@ -0,0 +1,26 @@
+# @TEST-DOC: A plugin testing some parts Zeek's C++ API
+# @TEST-EXEC: ${DIST}/auxil/zeek-aux/plugin-support/init-plugin -u . Demo API
+# @TEST-EXEC: cp -r %DIR/api-plugin/* .
+#
+# @TEST-EXEC: ./configure --zeek-dist=${DIST} && make
+#
+# @TEST-EXEC: ZEEK_PLUGIN_PATH=`pwd` zeek %INPUT >output
+#
+# @TEST-EXEC: TEST_DIFF_CANONIFIER= btest-diff output
+
+module DemoAPI;
+
+export {
+	type Severity: enum {
+		CRITICAL = 1,
+		ERROR = 2,
+		WARNING = 3,
+		INFO = 4,
+	};
+}
+
+module User;
+
+redef enum DemoAPI::Severity += {
+	USER_DEBUG = 50,
+};