Add new web testing framework

[elizabethcarney]

It would be awesome to add the new web testing stuff into the cookiecutter! The most important things to include are TestRunner.html, run_tests.sh, and a testing file stub.

A testing file stub could look a lot like this:

#include "../third-party/Empirical/source/web/Document.h"
#include "../third-party/Empirical/source/web/_MochaTestRunner.h"
struct ExampleTest : emp::web::BaseTest {
  int testValue;
  ExampleTest() { Setup(); }
  void Setup() {
    testValue = 2;
  }
  void Describe() override {
    EM_ASM({  
      const testValue = $0;
      describe("an example test", function() {
        it("should pass in a test value correctly", function() {
          chai.assert.equal(testValue, 2); 
        });
      });
    }, testValue);
  }
};
emp::web::MochaTestRunner test_runner;
int main() {
  emp::Initialize();
  test_runner.AddTest<ExampleTest>("ExampleTest");
  test_runner.Run();
}