Time zone should be explicitly set for timestamp related tests

[chiroptical]

If you change this to America/New_York the test will fail. My machine by default was set to that.

> rebar3 ct --suite=test/pgo_datetime_SUITE.erl
===> Verifying dependencies...
===> Analyzing applications...
===> Compiling pgo
===> Running Common Test suites...
%%% pgo_datetime_SUITE: ......
All 6 tests passed.

[chiroptical]

@tsloughter I can't reproduce that error anymore. Unsure exactly.

Next things on my list,

• Get types checking again
• Make the test environment less dependent on the docker-compose.yaml file

(note: this are completely separate from this PR but I had them on my mind for whatever reason)

[tsloughter]

Sounds good 👍