check_what.hpp: renamed check_what to check_what_equals, same for check_wwhat, added new check_wwhat_starts_with

Hopefully the purpose of the classes becomes more clear that way. Also,
the check_wwhat_starts_with is going to be required to make tests that
check error messages only partially (e.g. because they might end with a
localized part)
module_test: does not fail on non-english systems anymore (at the cost of not checking the system message anymore)