Don't pass shared_examples to fsc after rspec dry-run

Test-plan:
- view PS4, see that changes were made that would have caused
  shared examples to be queued up directly in FSC rather than
  from within their contexts
  - k5_dashboard_student_spec.rb:77 should be run by FSC
  but
  - k5_announcements_shared_examples.rb:193 should no longer be
  added

Change-Id: If909647c5efe10137b734f47a074365bcef3c944
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/274596
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Andrea Cirulli <andrea.cirulli@instructure.com>
QA-Review: Brian Watson <bwatson@instructure.com>
Product-Review: Brian Watson <bwatson@instructure.com>

Jenkinsfile.selenium.flakey_spec_catcher

@@ -97,6 +97,7 @@ def computeTestCount() {
       bash -c "bundle exec rspec --dry-run ${changedTestsArr.join(' ')} --require ./build/new-jenkins/rspec_location.rb --format RSpecLocationFormatter --out ${env.DOCKER_WORKDIR}/tmp/test_list_split"
   """
 
+  sh "sed -i '/shared_examples/d' $env.LOCAL_WORKDIR/tmp/test_list_split"
   changedTests = readFile("$env.LOCAL_WORKDIR/tmp/test_list_split").trim()
   changedTestsArr = changedTests.split('\n')