Test all backends when a runtest is modified (#5872)

* Test all backends when a runtest is modified

* Check that this triggers all backend tests

* Revert "Check that this triggers all backend tests"

This reverts commit 1d12536d04f5a3b01fa5420f407960d7ab81da8f.

ci/build-test-matrix.js

@@ -97,6 +97,13 @@ function myFilter(item) {
   if (item.filter && commits.includes(`prtest:${item.filter}`)) {
     return true;
   }
+
+  // If any runtest was modified, re-run the whole test suite as those can
+  // target any backend.
+  if (names.includes(`cranelift/filetests/filetests/runtests`)) {
+    return true;
+  }
+
   return false;
 }