Added a check in the job "test-store-config" for missing store field (BugFix)

[fernando79513]

Description

With the new template checks, we discovered an error with the "snappy/test-store-config-store" test. The test was trying to be created even if no store was declared.

CategorySnapdIO LogTemplate 'com.canonical.certification::snappy/test-store-config-store' generated an invalid unit. Field 'command' has value 'echo "Expected store ID:"
echo "$STORE_ID"
echo
echo "Store ID in model assertion:"
echo "{store}"
[ "$STORE_ID" == "{store}" ]' but the template-resource didn't generate any value for 'store'

Now we set the store field in the "model_resource" to "unknown" and filter that in the templated test

Resolved issues

#1663

N/A

Documentation

Tests

After running a reduced version of the snappy automated tests, the test-store-config test is not templated anymore

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 48.99%. Comparing base (f6f1b9b) to head (f1e78e5).
Report is 92 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1664   +/-   ##
=======================================
  Coverage   48.99%   48.99%           
=======================================
  Files         372      372           
  Lines       40319    40321    +2     
  Branches     6810     6811    +1     
=======================================
+ Hits        19755    19757    +2     
  Misses      19842    19842           
  Partials      722      722           

Flag	Coverage Δ
checkbox-support	∅ <100.00%> (∅)
provider-base	24.79% <ø> (ø)

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[Hook25]

tyvm for fixing this! +1