JS: Add: Test case for checking if regex via using splice

javascript/ql/test/library-tests/RegExp/IsRegex/IsRegex.expected

@@ -0,0 +1,2 @@
+| tst.js:1:12:1:38 | '^http: ... le.com' | is a regular expression |
+| tst.js:4:37:4:43 | 'regex' | is a regular expression |

javascript/ql/test/library-tests/RegExp/IsRegex/IsRegex.ql

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+import javascript
+
+from RegExpPatternSource regex
+select regex, "is a regular expression"

javascript/ql/test/library-tests/RegExp/IsRegex/tst.js

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+new RegExp('^http://test\.example.com'); // NOT OK
+
+function detectRegexViaSplice(string) {
+    let found = getMyThing().search('regex'); // NOT OK
+    arr.splice(found, 1);
+};