Register postgre driver in pkg/event/target (#6689)

Commit 5c13765168 removed postgre registration triggerd
by the automatic gofmt command but it was the only where pg is registered. This commit
fixes behavior and adds unit tests to check whether postgre & sql are registered or not.

vendor/github.com/lib/pq/TESTS.md

@@ -0,0 +1,33 @@
+# Tests
+
+## Running Tests
+
+`go test` is used for testing. A running PostgreSQL
+server is required, with the ability to log in. The
+database to connect to test with is "pqgotest," on
+"localhost" but these can be overridden using [environment
+variables](https://www.postgresql.org/docs/9.3/static/libpq-envars.html).
+
+Example:
+
+	PGHOST=/run/postgresql go test
+
+## Benchmarks
+
+A benchmark suite can be run as part of the tests:
+
+	go test -bench .
+
+## Example setup (Docker)
+
+Run a postgres container:
+
+```
+docker run --expose 5432:5432 postgres
+```
+
+Run tests:
+
+```
+PGHOST=localhost PGPORT=5432 PGUSER=postgres PGSSLMODE=disable PGDATABASE=postgres go test
+```