[STAL-1686] Follow-up for handling rules marked as is_testing

[modernplumbing]

What problem are you trying to solve?

When releasing version 0.3.3 of the analyzer, we started having 422s. This was due to the front-end not sending is_testing if is_testing was unset. The IDE integration would have the same issue. This PR does the following to address this issue:

1. Adds a default value (false) if is_testing is empty in the Rule struct.
2. Remove the is_testing field from the ServerRule struct.
3. Updates the test-production-rules.py integration test to expose the error if it fails, rather than just returned non-zero exit status 1.

• However, there could possibly be a cleaner way of surfacing the error. The current output is

Testing ruleset python-best-practices on the CLI
   Testing rule special-methods-arguments
Traceback (most recent call last):
  File "/home/runner/work/datadog-static-analyzer/datadog-static-analyzer/misc/test-production-rules.py", line 208, in test_ruleset_cli
    subprocess.run(shlex.split(cmd), check=True, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
  File "/usr/lib/python3.10/subprocess.py", line 526, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['/home/runner/work/datadog-static-analyzer/datadog-static-analyzer/target/release/datadog-static-analyzer', '-r', '/tmp/tmp7tcyto2m/special-methods-arguments/next-with-error.py.datadog.rules', '-i', '/tmp/tmp7tcyto2m/special-methods-arguments', '-o', '/tmp/tmp7tcyto2m/special-methods-arguments/next-with-error.py.results', '-f', 'json']' returned non-zero exit status 1.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/runner/work/datadog-static-analyzer/datadog-static-analyzer/misc/test-production-rules.py", line 243, in <module>
    test_ruleset_cli(ruleset)
  File "/home/runner/work/datadog-static-analyzer/datadog-static-analyzer/misc/test-production-rules.py", line 210, in test_ruleset_cli
    raise RuntimeError("command '{}' return with error (code {}): {}".format(e.cmd, e.returncode, e.output))
RuntimeError: command '['/home/runner/work/datadog-static-analyzer/datadog-static-analyzer/target/release/datadog-static-analyzer', '-r', '/tmp/tmp7tcyto2m/special-methods-arguments/next-with-error.py.datadog.rules', '-i', '/tmp/tmp7tcyto2m/special-methods-arguments', '-o', '/tmp/tmp7tcyto2m/special-methods-arguments/next-with-error.py.results', '-f', 'json']' return with error (code 1): b'Error: cannot read ruleset from file\n\nCaused by:\n    missing field `is_testing` at line 1 column 2486\n'

What is your solution?

Testing

Followed the procedure here: #339