add error tests for pyflakes rule

testdata/err/pyflakes_job_default_shell.out

@@ -0,0 +1 @@
+/test\.yaml:9:9: pyflakes reported issue in this script: .+ \[pyflakes\]/

testdata/err/pyflakes_job_default_shell.yaml

@@ -0,0 +1,9 @@
+on: push
+jobs:
+  job-level:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        shell: python
+    steps:
+      - run: print(unknown_var)

testdata/err/pyflakes_step_shell.out

@@ -0,0 +1,3 @@
+/test\.yaml:7:9: pyflakes reported issue in this script: .+ \[pyflakes\]/
+/test\.yaml:10:9: pyflakes reported issue in this script: .+ \[pyflakes\]/
+/test\.yaml:13:9: pyflakes reported issue in this script: .+ \[pyflakes\]/

testdata/err/pyflakes_step_shell.yaml

@@ -0,0 +1,14 @@
+on: push
+jobs:
+  step-level:
+    runs-on: ubuntu-latest
+    steps:
+      # Lint check failed. The error is output to stdout (#411)
+      - run: print(unknown_var)
+        shell: python
+      # Syntax check failed. The error is output to stderr (#411)
+      - run: print(
+        shell: python
+      # Check custom shell
+      - run: print(unknown_var)
+        shell: 'python {0}'

testdata/err/pyflakes_workflow_default_shell.out

@@ -0,0 +1 @@
+/test\.yaml:11:9: pyflakes reported issue in this script: .+ \[pyflakes\]/

testdata/err/pyflakes_workflow_default_shell.yaml

@@ -0,0 +1,11 @@
+on: push
+
+defaults:
+  run:
+    shell: python
+
+jobs:
+  job-level:
+    runs-on: ubuntu-latest
+    steps:
+      - run: print(unknown_var)