Reorganize GAPIC tests

[lukesneeringer]

Python smoke tests have recently been added to autogen. Yay!

They are being put in a place where they will never run, though, because the default test loader for unittest requires a test_ prefix for every file.

Right now the directory structure is:

test/
  google/
    cloud/
      {api}/
        {version}/
          {api}_smoketest.py
          test_{client}.py

This should become:

tests/  # Note the -s suffix.
  unit/
    gapic/
      {version}/
        test_{client}.py
  system/
    gapic/
      {version}/
        test_{api}.py

The split for unit/ and system/ is also critical because our test setup in CI knows when to run and not run system tests.

[lukesneeringer]

@landrito Assigning to you because I assign all auto-gen things to you. :-) But feel free to reassign if you are not the right person.

[lukesneeringer]

Also note the use of "system" and not "smoke". We consistently call them "system tests" in google-cloud-python.