Improve unit test coverage of CLI and Runners

[AdeelH]

Overview

This PR

• adds unit tests that invoke CLI commands
• adds a unit test that runs the tiny_spacenet example
• refactors Runners for clarity.

It also removes the steps in the release process that asked for testing the example pipelines, since that is now covered by unit tests.

Checklist

• Added needs-backport label if PR is bug fix that applies to previous minor release
• Ran scripts/format_code and committed any changes
• Documentation updated if needed
• PR has a name that won't get you publicly shamed for vagueness

Notes

The PR does not add unit tests for submitting jobs via AWSBatchRunner, since mocking Batch with all the job queues and job definitions is very non-trivial.

Testing Instructions

• See new unit tests.

Closes #1919

[codecov]

Codecov Report

Merging #1966 (ca70b22) into master (e3abaf4) will increase coverage by 3.71%.
The diff coverage is 79.36%.

@@            Coverage Diff             @@
##           master    #1966      +/-   ##
==========================================
+ Coverage   82.47%   86.18%   +3.71%     
==========================================
  Files         190      191       +1     
  Lines        9337     9362      +25     
==========================================
+ Hits         7701     8069     +368     
+ Misses       1636     1293     -343     

Files	Coverage Δ
...ervision/pytorch_backend/examples/tiny_spacenet.py	100.00% <100.00%> (ø)
...eline/rastervision/pipeline/runner/local_runner.py	98.30% <97.05%> (+80.30%)	⬆️
rastervision_pipeline/rastervision/pipeline/cli.py	93.68% <50.00%> (+60.35%)	⬆️
...s_batch/rastervision/aws_batch/aws_batch_runner.py	53.12% <58.33%> (+35.73%)	⬆️

... and 20 files with indirect coverage changes