rcutorture: Don't wait for kernel when all builds fail

Currently, rcutorture groups runs in batches, building each scenario in
a given batch, then invoking qemu to run all the kernels in the batch.
Of course, if a given scenario's kernel fails to build, there is no qemu
run for that scenario.  And if all of the kernels in a given batch fail
to build, there are no runs, and rcutorture immediately starts on the
next batch.

But not if --jitter has been specified, which it is by default.  In this
case, the jitter scripts are started unconditionally, and rcutorture
waits for them to complete, even though there are no kernels to run.

This commit therefore checks for this situation, and refuses to start
jitter unless at least one of the kernels in the batch built successfully.
This saves substantial time when all scenarios' kernels fail to build,
particularly if a long --duration was specified.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

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