service/dap: fix TestNextParked/TestStepInParked (#2643)
service/dap: fix TestNextParked/TestStepInParked (#2643)

The loop searching for a suitable goroutine is not guaranteed to ever
find it, and failing to find one is not an error.

Changes testStepParkedHelper to match the behavior of TestNextParked in
proc_test.go.
Deletes TestStepInParked because it does not test anything meaningful
beyond what's already tested by TestNextParked.
go-delve/delve,go-delve/delve,go-delve/delve,go-delve/delve,go-delve/delve