Regexp_engine::Reset() failed to initialize the status variable
m_error_code.  This could lead to an internal ICU warning generated
for one row "spilling" over to the processing of subsequent rows.

In Regexp_engine::Replace(), test for U_SUCCESS rather than U_ZERO_ERROR,
i.e. ignore warnings for the 'not found' case.

Add unittest to verify that warnings are reset between regexp engine
executions.

Change-Id: Ifdf70c33976228e86c0393d90a88da037ac595db