build(ci): Skip backend test job instead of each step (#24198)

Changes backed test workflow to skip at the job level instead of each job step.

This simplifies the workflow and more accurately reflects the status of backend tests when backend files change. See https://github.com/getsentry/getsentry/issues/5196 for more details.

.github/workflows/backend-test.yml

@@ -7,10 +7,38 @@ on:
   pull_request:
 
 jobs:
+  # If we make these jobs "required" to merge on GH, then on every PR, GitHub automatically
+  # creates a status check in the "pending" state. This means that the workflow needs to run
+  # for every PR in order to update the status checks.
+  #
+  # In order to optimize CI usage, we want the tests to only run when python files change,
+  # since frontend changes should have no effect on these test suites. We cannot use GH workflow
+  # path filters because entire workflow would be skipped vs skipping individual jobs which
+  # would still allow this status check to pass.
+  files-changed:
+    name: check backend files changed
+    runs-on: ubuntu-20.04
+    timeout-minutes: 5
+
+    outputs:
+      backend: ${{ steps.changes.outputs.backend }}
+
+    steps:
+      - uses: actions/checkout@v2
+      - name: Check for python file changes
+        uses: getsentry/paths-filter@v2
+        id: changes
+        with:
+          token: ${{ github.token }}
+          filters: .github/file-filters.yml
+
   test:
     name: backend test
+    needs: files-changed
     runs-on: ubuntu-20.04
-    timeout-minutes: 30
+    timeout-minutes: 20
+    if: needs.files-changed.outputs.backend == 'true'
+
     strategy:
       matrix:
         instance: [0, 1]
@@ -38,24 +66,20 @@ jobs:
 
       - name: Set python version output
         id: python-version
-        if: steps.changes.outputs.backend == 'true'
         run: |
           echo "::set-output name=python-version::$(cat .python-version)"
 
       # Until GH composite actions can use `uses`, we need to setup python here
       - uses: actions/setup-python@v2
-        if: steps.changes.outputs.backend == 'true'
         with:
           python-version: ${{ steps.python-version.outputs.python-version }}
 
       - name: Setup pip
         uses: ./.github/actions/setup-pip
-        if: steps.changes.outputs.backend == 'true'
         id: pip
 
       - name: pip cache
         uses: actions/cache@v2
-        if: steps.changes.outputs.backend == 'true'
         with:
           path: ${{ steps.pip.outputs.pip-cache-dir }}
           key: ${{ runner.os }}-py${{ steps.python-version.outputs.python-version }}-pip${{ steps.pip.outputs.pip-version }}-${{ hashFiles('**/requirements-*.txt') }}
@@ -64,13 +88,11 @@ jobs:
 
       - name: Setup sentry env
         uses: ./.github/actions/setup-sentry
-        if: steps.changes.outputs.backend == 'true'
         id: setup
         with:
           snuba: true
 
       - name: Run backend test (${{ steps.setup.outputs.matrix-instance-number }} of ${{ strategy.job-total }})
-        if: steps.changes.outputs.backend == 'true'
         run: |
           # Note: `USE_SNUBA` is not used for backend tests because there are a few failing tests with Snuba enabled.
           unset USE_SNUBA