Adding backend tests

[aaron-wu1]

• added additional power and teros tests
• added sensors tests
• added health check resource functionality test
• migrated db session code to be oop

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 99.43503% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
backend/api/models/sensor.py	50.00%	1 Missing :warning:

Flag	Coverage Δ
backend	74.14% <99.43%> (+14.84%)	:arrow_up:
frontend	76.00% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
backend/api/models/logger.py	90.00% <100.00%> (ø)
backend/api/models/power_data.py	79.66% <ø> (+30.50%)	:arrow_up:
backend/api/resources/cell.py	38.70% <ø> (+7.45%)	:arrow_up:
backend/api/schemas/add_cell_schema.py	100.00% <100.00%> (ø)
backend/tests/conftest.py	89.28% <100.00%> (+2.32%)	:arrow_up:
backend/tests/test_cell.py	100.00% <100.00%> (ø)
backend/tests/test_health_check.py	100.00% <100.00%> (ø)
backend/tests/test_json_encoder.py	100.00% <100.00%> (ø)
backend/tests/test_power_data.py	100.00% <100.00%> (ø)
backend/tests/test_sensor.py	100.00% <100.00%> (ø)
... and 2 more

... and 5 files with indirect coverage changes