InlineExpectationsTest: Add test showing what happens if you leave out getARelevantTag

python/ql/test/library-tests/InlineExpectationsTest/missing-relevant-tag/Test.expected

@@ -0,0 +1 @@
+| test.py:4:1:4:3 | foo | Unexpected result: foo=val |

python/ql/test/library-tests/InlineExpectationsTest/missing-relevant-tag/Test.ql

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+// test to illustrate what happens if you forget to put in the
+// right values for `getARelevantTag`. We want to alert on this,
+// so it gets fixed!
+import python
+import TestUtilities.InlineExpectationsTest
+
+class MissingRelevantTag extends InlineExpectationsTest {
+  MissingRelevantTag() { this = "MissingRelevantTag" }
+
+  override string getARelevantTag() { none() }
+
+  override predicate hasActualResult(Location location, string element, string tag, string value) {
+    exists(Name name | name.getId() = "foo" |
+      location = name.getLocation() and
+      element = name.toString() and
+      value = "val" and
+      tag = "foo"
+    )
+  }
+}

python/ql/test/library-tests/InlineExpectationsTest/missing-relevant-tag/test.py

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+foo # $ foo=val
+
+# with wrong value
+foo # $ foo=bad-value
+
+# there is a typo here, so this result is actually missing!
+fooo # $ foo=val