Fix execvpe ENOEXEC behavior.

The special case for absolute paths wasn't handling ENOEXEC.

Also add more extensive tests for execvpe.

Also switch to manually doing the fork in ExecTestHelper::Run because
ASSERT_EXIT doesn't actually return, meaning we were only running the
first part of each test.

Bug: http://b/31073104
Change-Id: I7a4640afc6d290c51ba2e66fc1b9bb6b0fc174f7