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

In the Mknod.parent_inode test case, the kernel sends an extra
FUSE_FORGET message. But because it gets sent asynchronously with the
failing syscall, it doesn't always get received before the test ends.
So we never setup an expectation for it. And 90+% of the time the test
would exit successfully.

Fix the intermittency by always waiting to receive the FUSE_FORGET
message.

Sponsored by: Axcient

(cherry picked from commit 86885b18689889e9b9142fd31d8c67f21334ba32)

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.

Sponsored by: Axcient

(cherry picked from commit 8399d764c929a4b2fa98dbfae0ca7359810e4668)