[lldb] Fix printf formatting of std::time_t seconds (#81078)

This formatter
https://github.com/llvm/llvm-project/pull/78609
was originally passing the signed seconds (which can refer to times in
the past) with an unsigned printf formatter, and had tests that expected
to see negative values from the printf which always failed on macOS. I'm
not clear how they ever passed on any platform.

Fix the printf to print seconds as a signed value, and re-enable the
tests.