Adjust error-message tests to pass with Clang

Clang emits full function signatures for __func__ where GCC emits
only the function name.
Adjust Gnucash's own error messages emitted via PWARN and friends
to trim the extra bits so that only the function name is emitted
regardless of compiler. For messages emitted directly by glib
(usually from g_return_if_fail macros), the message in the test
case is special-cased for Clang so that it matches and the test
passes.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23289 57a11ea4-9604-0410-9ed3-97b8803252fd