[demangler] improve test harness

The demangler test harness is a little unclear.  The failed demangling
message always causes me to think about 'reality', changing to a
simple 'Found' seems clearer.

The expected-to-fail tests abort as soon as one passes, rather than
continue, and then abort if any passed.  This changes that loop to
fail at the end, in a similar manner to the expected-to-work loop.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D118130