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.
BUG: Fix np.insert for inserting a single item into a structured array

Note that there are some object array special cases because of allowing
multiple inserts. `np.array(..., dtype=object)` is not always clear.
Merge pull request #5022 from seberg/structured-insert

BUG: Fix np.insert for inserting a single item into a structured array
Merge pull request #5020 from charris/disable-some-test-warnings

TST: Silence some warning that turns up on OpenBSD.