Fix assert_raises calls

[amomchilov]

Motivation

These calls are trying to pass a String/Regex as a matcher, but it's actually just interpreted as an exception type to try to rescue. It's confusing the base Minitest version of assert_raises(*exp) for the augment Rails assert_raises(*exp, match:).

[Morriar]

Looks like assert_match will create a regexp anyway:

    pattern = case(pattern)
      when String
        Regexp.new(Regexp.escape(pattern))
      else
        pattern
    end

Wouldn't passing a literal regexp with /pattern/ be slightly faster and more memory efficient?

[amomchilov]

AHaha i saw this change and had an inkling you might flag this. Won't someone think of the bytes?!

If it needs escaping, I'll keep the string format, because it's much easier to read without the backslashes (e.g. /sig \{ returns\(void\)\) \}/)