oeqa/selftest/buildoptions: Ensure diskmon tests run consistently

oeqa/selftest/buildoptions: Ensure diskmon tests run consistently

Heartbeat events default to once a second and we need to ensure we have
enough time in the task to see them.

Add a nostamp delay task 5s long so we can have a consistently timed
task which doesn't need cleanup or have unneeded dependencies. This
ensures we should deterministically see the disk moinitor events
regardless of the state of the build. This is done in a way which
doesn't corrupt build state or need cleanup and is efficient.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>