Add automated tests for plugins

Test infrastructure and sample confs/rules/traces for plugins
automated tests:

New test cases are in falco_tests_plugins.yaml and cover:
- Listing plugins and fields when plugins are loaded.
- Basic cloudtrail + json plugin on a fake cloudtrail json file and a
  sample rule that uses both plugins.
- Conflicts between source/extractor plugins
- Incompatible plugin api
- Wrong plugin path
- Checking for warnings when reading rules with unnown sources (e.g. when plugins are not loaded)

Some test-only plugins written in C are in test/plugins and built on
the fly. (They aren't included in packages of course).

The test framework needed some small changes to handle these tests:
- Add a mode to not check detection counts at all (for --list/--list-plugins)
- addl_cmdline_opts to allow specifying --list/--list-plugins
- Using DOTALL when matching stderr/stdout (allows multi-line matches more easily)

Signed-off-by: Mark Stemm <mark.stemm@gmail.com>

test/plugins/incompat_api.cpp

@@ -0,0 +1,28 @@
+/*
+Copyright (C) 2021 The Falco Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+#include <string.h>
+#include <stdlib.h>
+
+// Don't need any function other than plugin_get_required_api_version,
+// plugin load will fail after that.
+static const char *pl_required_api_version = "10000000.0.0";
+
+extern "C"
+const char* plugin_get_required_api_version()
+{
+	return pl_required_api_version;
+}