TST: Silence some warning that turns up on OpenBSD.

On OpenBSD with gcc-4.2 log1p(nan) raises an invalid value warning.
This PR disables both 'divide' and 'invalid' for the log1p tests.

Closes #5017.
Merge pull request #5020 from charris/disable-some-test-warnings

TST: Silence some warning that turns up on OpenBSD.