Merge branch 'ilya-fursov/ts-fixevent2' into 'main'

TSEvent: refactor and fix bugs, add TSSetPostEventStep()

See merge request petsc/petsc!6752

Implement setting two distinct post-event steps

The two post-event steps are managed by:
TSSetPostEventStep(), -ts_event_post_event_step,
TSSetPostEventSecondStep(), -ts_event_post_event_second_step

Implement setting two distinct post-event steps

The two post-event steps are managed by:
TSSetPostEventStep(), -ts_event_post_event_step,
TSSetPostEventSecondStep(), -ts_event_post_event_second_step.

(The latter function and option are replacement for the original PETSc
TSSetPostEventIntervalStep(), -ts_event_post_eventinterval_step,
with some change of overall behaviour of 1st + 2nd steps).

Also, reduce the number of tests ~ 4 times.

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