Memory Test for phet brand

[SaurabhTotey]

Standard code review issue. Created for #35. I'll perform the memory test and put the data here soon.

[SaurabhTotey]

First run: Tested using the built_all_debug version from master, SHA was 3a7c86f, common code was whatever was on master today, 07/13/2021.

Test	Memory Usage (MB)
Immediately after load, no fuzzing	22.1
Load, visit each screen, then back to home screen, no fuzzing	22.6
After 1 min of fuzz	27.8
After 2 min of fuzz	28.6
After 3 min of fuzz	28.6
After 4 min of fuzz	28.9
After 5 min of fuzz	29.2
Stop fuzz, reset each screen, go to home screen	29.3

This seems like it is a reasonable memory growth profile. With @jbphet, we compared the post-fuzz and pre-fuzz snapshots and saw no leakage of sim-specific classes. I will leave this issue open, and let @pixelzoom close at their discretion when doing the code review.

[pixelzoom]

I agree, looks reasonable. QA will verify again during dev/RC testing, as part of their standard procedures, so I won't spend time re-running tests. Closing.