tests: simplify t0 test and increase task queue usage

We now only wait for Done at the end, instead of in every call to
introspect.

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>
tests: fix a race condition in the toolchanger/toolno-pocket-differ test

It's a terrible idea to use sleep to synchronize two threads.  Better to
ask linuxcncrsh to wait for linuxcnc to finish.

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>
Merge remote-tracking branch 'origin/2.6' into 2.7