bb_test: split out the process exec logic

This allows us to run bitbake-whatchanged in the same way we run bitbake.

Signed-off-by: Christopher Larson <kergoth@gmail.com>
bb_test: let the user use a custom status message

Signed-off-by: Christopher Larson <kergoth@gmail.com>
leaky-layer-tests: add new test script

This is a simple script which uses a default configuration (poky/qemux86), and
adds each available bsp or distro layer to the configuration, one at a time,
monitoring for changes to the target with bitbake-whatchanged. This helps one
to identify bsp layers which affect machines other than the ones it provides,
and distro layers which affect other distros when included.

Signed-off-by: Christopher Larson <kergoth@gmail.com>