Improve resource/pagerduty_schedule open incidents handling on deletion

[imjaroiswebdev]

Complements #679

Some users where reporting that the inclusion of UserIDs as an additional query param for targeting the incidents was generating error on the retries for listing open incidents. So, this update is addressing that issue.

Additionally, I took the chance for moving the incident query to happen when a 400 error is received and it's also containing an expected message, to make the call for the open incidents, which turns out to be more efficient than making the call each time a Schedule was trying to be deleted.

Finally, I extended the test case covering the case when an unrelated incident get created and assigned to the same user in the Schedule that's going to be deleted, but the user received the incident assignment because of a different Escalation Policy, and this latter is not using the above mentioned Schedule.

Test cases extended...

$ make testacc TESTARGS="-run TestAccPagerDutySchedule"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run TestAccPagerDutySchedule -timeout 120m
?       github.com/terraform-providers/terraform-provider-pagerduty     [no test files]
=== RUN   TestAccPagerDutySchedule_import
--- PASS: TestAccPagerDutySchedule_import (12.50s)
=== RUN   TestAccPagerDutySchedule_Basic
--- PASS: TestAccPagerDutySchedule_Basic (15.48s)
=== RUN   TestAccPagerDutyScheduleWithTeams_Basic
--- PASS: TestAccPagerDutyScheduleWithTeams_Basic (16.05s)
=== RUN   TestAccPagerDutySchedule_BasicWithExternalDestroyHandling
--- PASS: TestAccPagerDutySchedule_BasicWithExternalDestroyHandling (12.86s)
=== RUN   TestAccPagerDutyScheduleWithTeams_EscalationPolicyDependant
--- PASS: TestAccPagerDutyScheduleWithTeams_EscalationPolicyDependant (17.81s)
=== RUN   TestAccPagerDutyScheduleWithTeams_EscalationPolicyDependantWithOpenIncidents
--- PASS: TestAccPagerDutyScheduleWithTeams_EscalationPolicyDependantWithOpenIncidents (50.52s)
=== RUN   TestAccPagerDutySchedule_EscalationPolicyDependantWithOpenIncidents # 👈 Test case extended
--- PASS: TestAccPagerDutySchedule_EscalationPolicyDependantWithOpenIncidents (51.28s)
=== RUN   TestAccPagerDutyScheduleOverflow_Basic
--- PASS: TestAccPagerDutyScheduleOverflow_Basic (15.35s)
=== RUN   TestAccPagerDutySchedule_BasicWeek
--- PASS: TestAccPagerDutySchedule_BasicWeek (14.95s)
=== RUN   TestAccPagerDutySchedule_Multi
--- PASS: TestAccPagerDutySchedule_Multi (15.26s)
PASS
ok      github.com/terraform-providers/terraform-provider-pagerduty/pagerduty   222.583s