[FIXED JENKINS-21999] If a slave node does not exist it will throw a
null pointer exception. Instead we create a dummy launcher and return
that if it is unable to get the slave node.
Throw IOException if trying to launch a DummyLauncher. Restrict
DummyLauncher class to no external use.
Do not redirectTestOutputToFile when we are running an individual test (suite or case).
In this case we are probably interested in looking at the output as it arrives in real time.
Can always be overridden on the command line if desired.
Merge branch 'JENKINS-21999' of github.com:christ66/jenkins
[JENKINS-21999] Noting merge of #1145.
OldDataMonitorTest.robustness consistently passes for me locally, but has been causing trouble in CI. Suppressing for now.
[JENKINS-22354]

Avoid having FilePath to depend on Jenkins, which increases the amount of classloading that has to happen during remoting
[JENKINS-22354]

Reduce classloader activities.

This brings in Stapler and commons bean-utils that we don't need on the
slaves.
fixed a test regression