Fixes flakes like the following:
```
workspaces_test.go:4938:
Error Trace: /home/runner/work/coder/coder/coderd/coderdtest/coderdtest.go:1279
/home/runner/work/coder/coder/coderd/workspaces_test.go:4938
/home/runner/work/coder/coder/coderd/workspaces_test.go:5044
Error: Received unexpected error:
POST http://127.0.0.1:42597/api/v2/users/me/workspaces: unexpected status code 409: Workspace "romantic-mcclintock" already exists.
name: This value is already in use and should be unique.
Test: TestWorkspaceCreateWithImplicitPreset/SinglePresetWithParameters
```
https://github.com/coder/coder/actions/runs/17142665868/job/48633017007?pr=19464
Which are caused by insufficient randomness when creating multiple
workspaces with random names. Two words is not enough to avoid flakes.
We have a `testutil.GetRandomName` function that appends a monotonically
increasing integer, but this alternative function that uses hyphens
doesn't add that integer. This PR fixes that by just
`testutil.GetRandomName`
Fixes#13910
Adds testutil.GetRandomName that replaces namesgenerator.GetRandomName but instead appends a monotonically increasing integer instead of a number between 1 and 10.