sh/tests: Fix flaky execution/bg12.0

When job control is not enabled, the shell ignores SIGINT while waiting for
a foreground process unless that process exits on SIGINT. In this case, the
foreground process is sleep and it does not exit on SIGINT because the
signal is only sent to the shell. Depending on order of events, this could
cause the SIGINT to be unexpectedly ignored.

On lightly loaded bare metal, the chance of this happening tends to be less
than 0.01% but with higher loads and/or virtualization it becomes more
likely.

Starting the sleep in background and using the wait builtin ensures SIGINT
will not be ignored.

PR: 247559
Reported by: lwhsu
MFC after: 1 week