[ticket/11615]  Rename continue -> check_isvalid for clarity

PHPBB3-11615
[ticket/11615] Refactored isvalid test to be more imperative

Refactoring the continue/is_valid test to remove
the confusing data provider work around, while
still keeping redundancies down to a minimum.

PHPBB3-11615
Merge commit '7293ac9b8c9effe8b3c623518326f6b71fb1255f' into ticket/11620
Merge commit '18caef66b794869d0089cdb4fe270c07e53b13c7' into ticket/11620