kvcoord: refactor ambiguous commit tests

In cockroachdb#107323, testing for the ambiguous write case that leads to the
"transaction unexpectedly committed" bug were introduced, however to
increase test coverage of the fix, multiple schedules of operations need
to be tested. This change simply refactors the framework of the existing
test in order to enable the addition of muliple subtests. The subtests
are included in a separate patch.

Part of: cockroachdb#103817

Release note: None