Do not match expectations outside specs

[HertzDevil]

Some calls of #should are done outside spec examples (more specifically, an it or before_each), which means if the expectation does not match, the Spec::AssertionFailed exception is not handled and will stop any subsequent specs from being added and run. This PR fixes those occurrences.

They are detected by monkeypatching Spec:

module Spec
  class_property? running_spec = false

  module ObjectExtensions
    def should(expectation, ...)
      STDERR.puts "expectation outside spec! #{file}:#{line}" unless Spec.running_spec?
      # ...
    end

    # ditto for the other 4 overloads of `#should` and `#should_not`
  end
end

Spec.before_each { Spec.running_spec = true }
Spec.after_each { Spec.running_spec = false }

[oprypin]

They are detected by monkeypatching Spec:

There's probably some value in making this permanent, maybe with a flag.
But that's just a side comment

[HertzDevil]

An assertion failure here is an unhandled exception for a signal handler, which would have been a fatal runtime error, so this spec requires some non-trivial editing.

[HertzDevil]

This one is special because this helper is used inside both its and before_alls, and assertion failures in the latter are not caught. In any case, since the backtick always returns a String, the original should be_truthy never raises, so it was incorrect in the first place.