Mutation test testing.js module (#1204)

Configure Stryker to mutation test the `testing.js` module. This is done
at the integration level because, like `index.js` it's an exposed
module.

In order to support this, the unit tests for injection strings were
lifted to integration tests in order to provide adequate integration
test coverage to guarantee 100% mutation coverage.

stryker.integration.config.js

@@ -3,7 +3,7 @@
 export default {
   coverageAnalysis: "perTest",
   inPlace: false,
-  mutate: ["index.js"],
+  mutate: ["index.js", "testing.js"],
   testRunner: "tap",
   tap: {
     testFiles: ["test/integration/**/*.test.js"],

test/integration/testing/commonjs.test.js

@@ -1,6 +1,6 @@
 /**
  * @overview Contains integration tests for the CommonJS version of the testing
- * implementation of Shescape.
+ * utilities provided with Shescape.
  * @license MIT
  */
 

test/integration/testing/functional.test.js

@@ -1,16 +1,27 @@
 /**
- * @overview Contains integration tests for the testing implementation of
+ * @overview Contains integration tests for the testing utilities provided with
  * Shescape.
  * @license MIT
  */
 
 import { testProp } from "@fast-check/ava";
+import test from "ava";
 import * as fc from "fast-check";
 
 import { arbitrary } from "../_.js";
 
 import { Shescape } from "shescape";
-import { Shescape as Stubscape } from "shescape/testing";
+import { injectionStrings, Shescape as Stubscape } from "shescape/testing";
+
+test("injection strings", (t) => {
+  t.true(Array.isArray(injectionStrings));
+  t.true(injectionStrings.length > 0);
+
+  for (const injectionString of injectionStrings) {
+    t.is(typeof injectionString, "string");
+    t.true(injectionString.length > 0);
+  }
+});
 
 testProp(
   "escape (stubscape ~ shescape)",