Expand TestHelper to allow non-underscore prefixes

HLint 1.8.28 requires us to always add "ignore CamelCase", which can
be problematic sometimes (e.g. when using OverloadedStrings). Let's
expand TestHelper to also support less-standard 'caseFooBar' names, so
that we can remove the annotations.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>