like() and unlike() no longer warn about undef.

Not much sense, they can see it in the diagnostics, and the warning
message is a bit confusing.

    Use of uninitialized value $this in pattern match (m//) at x:\test.t line 6.

It reports the right location, but there's no $this from the user's
perspective.

This came in because like used to supress all warnings.  That was
fixed in 8b2f0c6e1fb789b0ea5ed217b5be18705bbe6b8c but it let
uninit warnings slip in.

Did it against master because it will merge nicely into Test-Builder1.5.

For #335.