Fix test cases to work with Semgrep constant propagation

Semgrep will propagate string constants (docs), so we must use actual variables or user input to trigger rules that detect non-hardcoded-string inputs.

For example, eslint.detect-non-literal-regexp has the following definition

patterns:
- pattern: |
    new RegExp(...)
- pattern-not: |
    new RegExp("...", ...)

To see this rule fire, we must change the test case from

var myregexpText = "/abcd/";
var myregexp = new RegExp(myregexpText);

to

function dangerous_regexp(myregexpText) {
  var myregexp = new RegExp(myregexpText);
}