Switch to modern RT::Test infrastructure for testing plugins

This side-steps incorrect test plan counts due to new bundled tests in
RT::Test itself.
Unset EmailAddress by setting the empty string rather than undef

RT doesn't support undef → NULL conversion yet, and everything in RT
expects a defined value.  The empty string is the canonical "no email
address" value.  In the future once RT moves to real NULLs, we can
revert this commit.
Record the merged/mergee's user Name if EmailAddress is empty

This avoids uninitialized in concatenation warnings and is more useful
than a blank in user comments.
Switch to .in files for substitution so as not to trash version-controlled files