[ddc] fix flaky for-loop test.

This test was flaky about 3% of the time in linux bots due to a race
condition.  Details are shared in
https://github.com/dart-lang/sdk/issues/55299. TLDR, when one test case
completes and it's execution is resumed (program 1),  the test driver
sets up a breakpoint for a new test case concurrently and schedules to
run main again (program 2). Depending on when the breakpoint is set, it
could be hit by either program 1 or program 2.  If both programs are in
the same state when they hit the breakpoint, then it doesn't matter
which program we use to validate the test expectations (a benign race
condition).  However, this is problematic when the state is different,
which can happen when test 1 and test 2 both are using the same
breakpoint in a loop. This is what happened for the `forLoopTestBP` in
this CL.

To workaround this data race between two test cases, I decided to simply
combine them into a single test case. A different alternative is to copy
the test function and use a different breakpoint, but that didn't seem
worthwhile in this case.


Change-Id: I561c2a25d33d56a8543fe061342cab9cceafd4e4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/359721
Reviewed-by: Nicholas Shahan <nshahan@google.com>
Commit-Queue: Sigmund Cherem <sigmund@google.com>