variable_watch: Add missing string enumeration entry (#14188)

In commit 9a5b4eba (All variable accesses should produce watch
callbacks, 2007-05-17) we added a new enumeration value but did not
update the list of strings matching them.  Add the missing entry.
Without this, variable_watch prints REMOVED_ACCESS instead of
MODIFIED_ACCESS when set the value of the variable.
variable_watch: Add test for MODIFIED_ACCESS report

Add a RunCMake.variable_watch watch test to cover the basic use case.
Merge topic 'fix-variable_watch-cases' into next

c63d300 variable_watch: Add test for MODIFIED_ACCESS report
3d5bb38 variable_watch: Add missing string enumeration entry (#14188)