Move default handlers from acceptance/internal to libs/testserver (#3495)

## Changes
- Move acceptance/internal/handlers.go to libs/testserver/handlers.go
- Pure move, no other changes.

## Why
The testserver package is useful outside of acceptance test as an
alternative to mocking the client. Default handlers could be useful as
well. I plan add a few unit tests for terranova/resources and those
require a client.

Author: denik

acceptance/internal/prepare_server.go

@@ -27,7 +27,7 @@ import (
 
 func StartDefaultServer(t *testing.T) {
 	s := testserver.New(t)
-	addDefaultHandlers(s)
+	testserver.AddDefaultHandlers(s)
 
 	t.Setenv("DATABRICKS_DEFAULT_HOST", s.URL)
 
@@ -174,7 +174,7 @@ func startLocalServer(t *testing.T,
 	}
 
 	// The earliest handlers take precedence, add default handlers last
-	addDefaultHandlers(s)
+	testserver.AddDefaultHandlers(s)
 	return s.URL
 }