Fix trend binary sensor and tests

[awarecan]

Description:

Trend binary sensor has the capability to calculate the state base one trend line. It is time-sensitive calculation, so the sample time should be the update_time of the entity's state, not the time trend sensor received sample.

Also, the test also time-sensitive, it may become flaky since everytime I run I got different numbers. This PR changed test to control the timing. Now I get more stable result.

One flaky test result I spotted at here: https://circleci.com/gh/home-assistant/home-assistant/3728#tests/containers/1

Related issue (if applicable): fixes #

Pull request in home-assistant.io with documentation (if applicable): home-assistant/home-assistant.io#<home-assistant.io PR number goes here>

Example entry for configuration.yaml (if applicable):

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[balloob]

Flaky test is homekit controller, fixed in another branch.