Add path filter to azure pipelines trigger

This restricts the pipeline from running on 'non-code' pushes and PRs in
the same way that GitHub workflows which run project tests are (although
the syntax is different, and must be made more explicit as wildcards
aren't supported).

azure-pipelines.yml

@@ -6,6 +6,28 @@ trigger:
     - 'master'
     # Release branches
     - '0.*'
+  paths:
+    include:
+      - 'mesonbuild'
+      - 'test cases'
+      - 'azure-pipelines.yml'
+      - 'ci/azure-steps.yml'
+      - 'run_project_tests.py'
+      - 'run_tests.py'
+      - 'run_unittests.py'
+pr:
+  branches:
+    include:
+    - '*'
+  paths:
+    include:
+      - 'mesonbuild'
+      - 'test cases'
+      - 'azure-pipelines.yml'
+      - 'ci/azure-steps.yml'
+      - 'run_project_tests.py'
+      - 'run_tests.py'
+      - 'run_unittests.py'
 
 variables:
   CI: 1