test: Add dbtest package

This is modeled after the internal/docker package to make it easy to use as a
replacement. Instead of creating a new docker container for each test,
it will create a new database in an existing database container, using a
template database as a source. This assumes that the template database
exists and already has the schema migrations run against it.

The default template is `boundary_template` which should already have
all migrations applied. However, some tests require an empty database to
test the migration code. In these cases the `template1` template
database should be used, ie:

    dbtest.StartWithTemplate("postgres", dbtest.WithTemplate("template1"))

testing/dbtest/docs.go

@@ -0,0 +1,25 @@
+// Package dbtest provides a way to create a clean database for tests using
+// template databases.  For it to function properly a postgres database server
+// must be running with the expected templates.  This is typically done using
+// the docker image provided in the docker directory of this package.
+//
+// To use this package in a test add something like the following in the
+// beginning of the test:
+//
+//  c, u, _, err := dbtest.StartUsingTemplate(dbtest.Postgres)
+//  require.NoError(t, err)
+//  t.Cleanup(func() {
+//    require.NoError(t, c())
+//  })
+//  // use u to get a connection to the new database
+//  dBase, err := sql.Open("postgres", u)
+//
+// By default this uses a template that already has all of the boundary
+// migrations run. If a test needs a database without any migrations, like in
+// the case where the migration code needs to be tested, a different template
+// can be specified. This should generally be template1:
+//
+//  c, u, _, err := dbtest.StartUsingTemplate(dbtest.Postgres, dbtest.WithTemplate(dbtest.Template1))
+//
+// See https://www.postgresql.org/docs/13/manage-ag-templatedbs.html
+package dbtest