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/test_extract.cpp

@@ -0,0 +1,116 @@
+/*
+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 <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <plugin_info.h>
+
+static const char *pl_required_api_version = "0.1.0";
+static uint32_t    pl_type                 = TYPE_EXTRACTOR_PLUGIN;
+static const char *pl_name_base            = "test_extract";
+static char pl_name[1024];
+static const char *pl_desc                 = "Test Plugin For Regression Tests";
+static const char *pl_contact              = "github.com/falcosecurity/falco";
+static const char *pl_version              = "0.1.0";
+static const char *pl_extract_sources      = "[\"test_source\"]";
+static const char *pl_fields               = "[]";
+
+// This struct represents the state of a plugin. Just has a placeholder string value.
+typedef struct plugin_state
+{
+} plugin_state;
+
+extern "C"
+const char* plugin_get_required_api_version()
+{
+	return pl_required_api_version;
+}
+
+extern "C"
+uint32_t plugin_get_type()
+{
+	return pl_type;
+}
+
+extern "C"
+const char* plugin_get_name()
+{
+	// Add a random-ish suffix to the end, as some tests load
+	// multiple copies of this plugin
+	snprintf(pl_name, sizeof(pl_name)-1, "%s%ld\n", pl_name_base, random());
+	return pl_name;
+}
+
+extern "C"
+const char* plugin_get_description()
+{
+	return pl_desc;
+}
+
+extern "C"
+const char* plugin_get_contact()
+{
+	return pl_contact;
+}
+
+extern "C"
+const char* plugin_get_version()
+{
+	return pl_version;
+}
+
+extern "C"
+const char* plugin_get_extract_event_sources()
+{
+	return pl_extract_sources;
+}
+
+extern "C"
+const char* plugin_get_fields()
+{
+	return pl_fields;
+}
+
+extern "C"
+const char* plugin_get_last_error(ss_plugin_t* s)
+{
+	return NULL;
+}
+
+extern "C"
+ss_plugin_t* plugin_init(const char* config, int32_t* rc)
+{
+	// Note: Using new/delete is okay, as long as the plugin
+	// framework is not deleting the memory.
+	plugin_state *ret = new plugin_state();
+	*rc = SS_PLUGIN_SUCCESS;
+	return ret;
+}
+
+extern "C"
+void plugin_destroy(ss_plugin_t* s)
+{
+	plugin_state *ps = (plugin_state *) s;
+
+	delete(ps);
+}
+
+extern "C"
+int32_t plugin_extract_fields(ss_plugin_t *s, const ss_plugin_event *evt, uint32_t num_fields, ss_plugin_extract_field *fields)
+{
+	return SS_PLUGIN_SUCCESS;
+}