pkg/term: refactor TestEscapeProxyRead
pkg/term: refactor TestEscapeProxyRead

- use subtests to make it clearer what the individual test-cases
  are, and to prevent tests from depending on values set by the
  previous test(s).
- remove redundant messages in assert (gotest.tools already prints
  a useful message if assertions fail).

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

moby/term