FIX: Appropriately handle uninstalled problem checks (#28771)

When running checks, we look to the existing problem check trackers and try to grab their ProblemCheck classes.

In some cases this is no longer in the problem check repository, e.g. when the check was part of a plugin that has been uninstalled.

In the case where the check was scheduled, this would lead to an error in one of the jobs

spec/models/problem_check_tracker_spec.rb

@@ -1,8 +1,6 @@
 # frozen_string_literal: true
 
 RSpec.describe ProblemCheckTracker do
-  before { described_class.any_instance.stubs(:check).returns(stub(max_blips: 1, priority: "low")) }
-
   describe "validations" do
     let(:record) { described_class.new(identifier: "twitter_login") }
 
@@ -24,6 +22,23 @@ RSpec.describe ProblemCheckTracker do
     end
   end
 
+  describe "#check" do
+    before do
+      Fabricate(:problem_check_tracker, identifier: "twitter_login")
+      Fabricate(:problem_check_tracker, identifier: "missing_check")
+    end
+
+    context "when the tracker has a corresponding check" do
+      it { expect(described_class[:twitter_login].check.new).to be_a(ProblemCheck) }
+    end
+
+    context "when the checking logic of the tracker has been removed or renamed" do
+      it do
+        expect { described_class[:missing_check].check }.to change { described_class.count }.by(-1)
+      end
+    end
+  end
+
   describe "#ready_to_run?" do
     let(:problem_tracker) { described_class.new(next_run_at:) }
 
@@ -169,7 +184,10 @@ RSpec.describe ProblemCheckTracker do
     context "when there are still blips to go" do
       let(:blips) { 0 }
 
+      before { ProblemCheck::TwitterLogin.stubs(:max_blips).returns(1) }
+
       it "does not sound the alarm" do
+        puts ProblemCheck::TwitterLogin.max_blips
         expect { problem_tracker.problem!(next_run_at: 24.hours.from_now) }.not_to change {
           AdminNotice.problem.count
         }