refactor: migrate login_linux_test.go to nerdtest.Setup

Replace testutil.NewBase-based test helpers with the Tigron test
framework (nerdtest.Setup). Key changes:

- Client.Run(base, host) replaced with Client.Cmd(helpers, host)
  returning test.TestableCommand; DOCKER_CONFIG set via Setenv.
- testregistry.NewRegistry / testca.New replaced with
  nerdtest.RegistryWithBasicAuth, nerdtest.RegistryWithTokenAuth, and
  lower-level registry.NewCesantaAuthServer + registry.NewDockerRegistry
  for the token-auth/no-TLS case (HTTP registry, matching original
  behaviour).
- TestLoginPersistence: two SubTests (basic, token) each owning their
  registry lifecycle via Setup/Cleanup.
- TestLoginAgainstVariants: dynamically generated SubTests from the
  existing test-case table; inner regHost and assertion loops run
  sequentially inside Setup (equivalent coverage without nested t.Run).
- testutil.DockerIncompatible replaced with require.Not(nerdtest.Docker).
- TestAgainstNoAuth remains commented out, unchanged.

Closes #4613

Signed-off-by: Ogulcan Aydogan <ogulcanaydogan@hotmail.com>

Author: ogulcanaydogan