Reset highwater mark *before* rolling back

I didn't follow the recommended test plan. I created a unit test instead. Let me know if that isn't acceptable.

1) Drupal\Tests\migrate\Unit\MigrateSourceTest::testPreRollback
Expectation failed for method name is "set" when invoked 1 time.
Method was expected to be called 1 time, actually called 0 times.
FAILURES!
Tests: 13, Assertions: 37, Failures: 1.
Exiting with EXIT_CODE=1

Shows the test coverage is there.

Migration is not my best but going off @quietone in #16 mentioning this is still valid as a migration maintainer.
Solution matches the MR

Believe this one is good, but if wrong I apologize.

Backported to 11.1.x as a bug fix.

Committed and pushed 420f7b2ff2d to 11.x and 180597712ad to 11.2.x and a94e3c35cb7 to 11.1.x. Thanks!

Automatically closed - issue fixed for 2 weeks with no activity.