fix(junit): fix locale sensitivity with junit

[JonnyPower]

closes #323

What does this PR do?

Changes the start date format util to follow predictable ISO standard

What issues does this PR fix or reference?

323

Functionality Before

Test summary start date was in use locale

Functionality After

Test summary start date is in ISO format so can be used programmatically later by junit reporter

[JonnyPower]

can replicate with this test in junitReporter.test.ts;

  it('should format start time with different locale', () => {
    const modified = { ...testResults };
    modified.summary.testStartTime = `${new Date().toDateString()} ${new Date().toLocaleTimeString(
      'en-CA'
    )}`;
    expect(modified.summary.testStartTime).to.contain('.m.');
    reporter.format(modified);
  });

[shetzel]

We should be able to close this since https://github.com/forcedotcom/salesforcedx-apex/pull/334 was merged. Thanks for your hard work and patience with this @JonnyPower !