tests/run-coredump-unwind: Skip test if no coredump has been created
tests/run-coredump-unwind: Skip test if no coredump has been created

In some build environments, coredumps are not created even if the
corresponding ulimit is positive. This change skips the test if
the coredump is missing.

libunwind/libunwind,libunwind/libunwind,libunwind/libunwind