The `-k '-expr'` syntax is an old alias to `-k 'not expr'`. It's also
not a very convenient to have syntax that start with `-` on the CLI.
Deprecate it and suggest replacing with `not`.
---
The `-k 'expr:'` syntax discards all items until the first match and
keeps all subsequent, e.g. `-k foo` with
test_bar
test_foo
test_baz
results in `test_foo`, `test_baz`. That's a bit weird, so deprecate it
without a replacement. If someone complains we can reconsider or devise
a better alternative.
Ran Benita