test: Make test tolerant of warnings in the returned diagnostics

Prior to this change the strings.Contains assertion was only receiving a string representing the warning, and the error diagnostic was no longer being asserted against.

Author: SarahFrench

internal/configs/module_test.go

@@ -551,7 +551,12 @@ func TestModule_conflicting_backend_cloud_stateStore(t *testing.T) {
 				t.Fatal("module should have error diags, but does not")
 			}
 
-			if got := diags.Error(); !strings.Contains(got, tc.wantMsg) {
+			if tc.allowExperiments {
+				if len(diags) != 2 && len(diags.Errs()) != 1 {
+					t.Fatalf("expected 2 diagnostics (1 error, 1 warning), but got: %#v", diags)
+				}
+			}
+			if got := diags.Errs()[0]; !strings.Contains(got.Error(), tc.wantMsg) {
 				t.Fatalf("expected error to contain %q\nerror was:\n%s", tc.wantMsg, got)
 			}
 		})