Moved property tests to the main suite

Property tests can only run in Node, so they were previously in another
directory that only gets run in Node. Now they're next to the related
non-property tests and marked pending in the browser. This makes it more
likely that a developer who normally only runs tests in the browser will
notice and run them.

spec/helpers/requireFastCheck.js

@@ -0,0 +1,16 @@
+(function(env) {
+  var NODE_JS =
+    typeof process !== 'undefined' &&
+    process.versions &&
+    typeof process.versions.node === 'string';
+
+  env.requireFastCheck = function() {
+    if (!NODE_JS) {
+      env.pending(
+        "Property tests don't run in the browser. Use `npm test` to run them."
+      );
+    }
+
+    return require('fast-check');
+  };
+})(jasmine.getEnv());