Specify fwts data directory for snap checkbox (Bugfix)

[tomli380576]

(I accidentally broke #2252 during rebase xD this PR is a copy of that based on the current main)

Description

This PR fixes #1997 by specifying the -j option in fwts when we detect that we are in a snap environment.

Resolved issues

1. Snap and Debian checkbox produces inconsistent fwts klog result #1997
2. Fixed a few unbound variables and undefined variable access

Documentation

Basically we are telling fwts to read the directory in checkbox runtime instead of whatever /usr/share/fwts is inside a snap.

-j, --json-data-path         Specify path to fwts json data files - default is /usr/share/fwts

Tests

Original unit tests

Real hardware with snaps built here:

• https://certification.canonical.com/hardware/201812-26713/submission/472544/ (18.04 classic frontend)
• https://certification.canonical.com/hardware/201812-26713/submission/472576/ (24.04 classic frontend)
  The tests failed from real fwts errors, but fwts was correctly invoked with -j

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 55.32%. Comparing base (958af61) to head (21d333c).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
...kbox-support/checkbox_support/scripts/fwts_test.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2323      +/-   ##
==========================================
+ Coverage   55.24%   55.32%   +0.07%     
==========================================
  Files         413      413              
  Lines       44448    44618     +170     
  Branches     8194     8247      +53     
==========================================
+ Hits        24556    24683     +127     
- Misses      19032    19073      +41     
- Partials      860      862       +2     

Flag	Coverage Δ
checkbox-support	71.39% <90.00%> (+0.50%)	⬆️
provider-base	32.10% <100.00%> (-0.08%)	⬇️
provider-certification-client	57.14% <ø> (ø)

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Copilot]

Pull request overview

Fixes fwts execution inside snap-based checkbox by ensuring fwts reads its JSON data files from the checkbox runtime, and adds regression coverage for the new behavior.

Changes:

• Added get_fwts_base_cmd() to select the correct fwts invocation (snap vs deb).
• Updated fwts command construction in main() to use the new base command.
• Added unit tests covering snap/deb environment command selection and error behavior.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 8 comments.

File	Description
checkbox-support/checkbox_support/scripts/fwts_test.py	Introduces snap-aware fwts base command and uses it when building fwts invocations.
checkbox-support/checkbox_support/scripts/tests/test_fwts_test.py	Adds unit tests validating base command selection and snap-directory error handling.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

The SystemExit message is a bit hard to read due to string concatenation and the extra space inside "but '{}' ". Consider simplifying into a single formatted string and making it more actionable (e.g., explicitly stating the expected directory and hinting that the snap/checkbox-runtime may be corrupted or missing the fwts data files). This will also make tests/assertions cleaner.

Suggested change

	"We are in a snap environment, "
	+ "but '{}' ".format(fwts_json_data_dir)
	+ "doesn't exist"
	"FWTS JSON data directory '{}' does not exist under the "
	"CHECKBOX_RUNTIME snap directory. The snap/checkbox-runtime "
	"may be corrupted or missing the FWTS data files.".format(
	fwts_json_data_dir
	)