Optimise test suite speed

[tburrows13]

I noticed that some tests were taking far longer than they needed to. This was usually doing write_videofile on a long clip.

Here's the speed differences on my laptop between before and after these changes

Units are seconds
Total:
1m43s > 53.9

test_PR.py:
3.8 -> 2.9

test_Videos.py:
3.2 -> 0.3

test_examples.py:
4.6 -> 1.2

test_loop():
26.8 -> 2.5

test_resize():
2.8 -> 0.7

test_issues.py:
15.6 -> 5.1

test_subtitles():
11.2 -> 7.4

[coveralls]

Coverage remained the same at 66.018% when pulling 0839e70 on tburrows13:optimise-tests into 646674c on Zulko:master.

[mondeja]

Great! I'm seeing all fine 👍