These activities may already be running, and deleting them would cause
validation node to violate activity dependencies. For example,
already-running indexation activities will not be waited upon, causing the
unindexation activity spawned below to potentially finish before ongoing
indexation, leading to a catalog persistently inconsistent with object
database.
If these activities are not already running, then CMFActivity will anyway
discard them upon discovering that their execution context does not exist.
And if these activities are failed, then it is a site maintenance issue
(one should not keep failed activities for too long) and must not be a
common case, and not really worthy of handling here.
Also, rework a CMFActivity test checking that activties already-spawned on
a later-deleted context get discarded instead of being executed. Previous
test implementation order was to work around the activity deletion
removed in this commit. The implementation used here is more natural.