Add asymmetric equality tester to match a string against a regexp

- Also move the asymmetric testers into their own dir for easier
  locating.

[#58120558] Fix #243

spec/core/asymmetric_equality/StringMatchingSpec.js

@@ -0,0 +1,21 @@
+describe("StringMatching", function() {
+  it("matches a string against a provided regexp", function() {
+    var matcher = new j$.StringMatching(/foo/);
+
+    expect(matcher.asymmetricMatch('barfoobaz')).toBe(true);
+    expect(matcher.asymmetricMatch('barbaz')).toBe(false);
+  });
+
+  it("matches a string against provided string", function() {
+    var matcher = new j$.StringMatching('foo');
+
+    expect(matcher.asymmetricMatch('barfoobaz')).toBe(true);
+    expect(matcher.asymmetricMatch('barbaz')).toBe(false);
+  });
+
+  it("raises an Error when the expected is not a String or RegExp", function() {
+    expect(function() {
+      new j$.StringMatching({});
+    }).toThrowError(/not a String or a RegExp/);
+  });
+});