Currently, the documentation makes it sound like the test subcommand's filter
flag could accept some kind of pattern matching value like a glob or a regex,
although the function "createFilterFn" accepts a regex as an argument, there's
no way to pass an actual regex value from the CLI.
This commit makes it possible to pass a string that could be cast as regex
when string matches "^/.*/$".
With this change, a user can use the filter flag as follow:
deno test --filter "/test-.+/"
Also tested that `\` get escaped properly, on MacOS at least, and this is
also a valid flag:
deno test --filter "/test-\d+/"
