Fixing start-time and end-time related Jasmine tests.

**Note**

Could not reproduce the failing tests. Performed small refactoring.

**Description**

The following two tests are flaky on master:

1.) JavaScript.firefox.VideoControl constructor with start-time: saved position is 0, timer slider and VCR set to start-time (from javascript)

*Stacktrace*

    timeout: timed out after 5000 msec waiting for duration is set

2.) JavaScript.firefox.VideoControl constructor with end-time: saved position is greater than end-time, timer slider and VCR set to 0:00 (from javascript) 

*Stacktrace*

    timeout: timed out after 5000 msec waiting for duration is set

[ 1 ] https://jenkins.testeng.edx.org/job/edx-all-tests-auto-master/1545/SHARD=1,TEST_SUITE=unit/testReport/junit/JavaScript/firefox/VideoControl_constructor_with_start_time__saved_position_is_0__timer_slider_and_VCR_set_to_start_time/
[ 2 ] https://jenkins.testeng.edx.org/job/edx-all-tests-auto-pr/8683/SHARD=1,TEST_SUITE=unit/testReport/junit/JavaScript/firefox/VideoControl_constructor_with_end_time__saved_position_is_greater_than_end_time__timer_slider_and_VCR_set_to_0_00/

**Jira issue**

[BLD-1013](https://edx-wiki.atlassian.net/browse/BLD-1013).

**Reviewers**

- @polesye