Fix intermittency in the sys.fs.fusefs.symlink.main test

This change is identical to 86885b18689 but for symlink instead of
mknod. The kernel sends a FUSE_FORGET asynchronously with the final
syscall. The lack of an expectation caused this test to occasionally
fail.

Also, remove a sleep that accidentally snuck into a different test.

MFC after: 2 weeks
MFC with: 86885b18689889e9b9142fd31d8c67f21334ba32
Sponsored by: Axcient