test: convert some more tests to use fixtures

test: convert some more tests to use fixtures

Continue the conversion from use of globals (the config module) to
fixtures. If a program (like wmem_test or tshark) is unavailable, it
will be skipped now rather than failing the test.

The general conversion pattern is:
- Decorate each class with `@fixtures.uses_fixtures` and (for tests that
  run tshark) `@fixtures.mark_usefixtures('test_env')`.
- Convert all `config.cmd_*` to `cmd_*` and add an argument.
- Convert all `config.*_dir` to `dirs.*_dir` and add an argument.
- Convert users of `os.path.join(dirs.capture_file, ...)` to use a new
  'capture_file' fixture to reduce boilerplate code. Inline variables if
  possible (this conversion was done in an automated way using regexes).

Some other changes: tests that do not require a test environment (like
wmem_test) will use 'base_env' which avoids copying config files,
`env=config.test_env` got removed since this is the default. Some test
classes in suite_clopts were combined. Removed unused imports.

Change-Id: Id5480ffaee7d8d56cf2cb3189a38ae9afa7605a1
Reviewed-on: https://code.wireshark.org/review/30591
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot
Reviewed-by: Anders Broman <a.broman58@gmail.com>