Fix index for screenshots in HTML report

[mkj-is]

The enlarging of screenshots does not work correctly if there were more screenshots taken in one test. Opening any screenshot will enlarge the first one, since wrong index (which is always zero) is used.

This PR should fix the issue.

I am not sure how to test the template, since this is JS and HTML issue so writing simple behavior test would be pretty hard.

[mkj-is]

The test failed due to slow execution, I believe this is due to flakiness of this test. The change made in the pull request should not influence anything.

Then the performance should be way faster than running cat # features/steps/xcpretty_steps.rb:27
      XCPretty run time: 2.013216262
expected: < 2
      got:   2.013216262 (RSpec::Expectations::ExpectationNotMetError)

[mkj-is]

Closing this one due to no response from code owners.