When the string transformation to number failed, all following
transformation failed too.
This happend because status in decNumberFromString function is
updated just in error case. Reusing the DEC_CONTEXT that failed
before results into error even if the string is valid number.
Keep a cached copy of the man tests that we can use when no manpage
changes are made. This allows automated systems that might not have
easy access to a pipenv to build and run tests.
