Fix GAMSmodel quiet= behaviour

[khaeru]

• Close #404.
• As a temporary mitigation for #403, disable the "R CMD check" step in the "pytest" CI workflow.

How to review

No review: simple bugfix and CI changes only.

PR checklist

• [x] Continuous integration checks all ✅
• [x] ~Add or expand tests;~ coverage checks both ✅
• ~Add, expand, or update documentation.~
• ~Update release notes.~ Correction to new behaviour that has not been released yet.

[codecov[bot]]

Codecov Report

Merging #405 (5f50de5) into master (40ad9c2) will decrease coverage by 0.0%. The diff coverage is 100.0%.

@@           Coverage Diff            @@
##           master    #405     +/-   ##
========================================
- Coverage    96.8%   96.8%   -0.1%     
========================================
  Files          37      37             
  Lines        4251    4247      -4     
========================================
- Hits         4117    4113      -4     
  Misses        134     134             

Impacted Files	Coverage Δ
ixmp/model/gams.py	100.0% <100.0%> (ø)
ixmp/testing.py	79.2% <100.0%> (-0.6%)	:arrow_down:
ixmp/tests/core/test_scenario.py	100.0% <100.0%> (ø)
ixmp/tests/reporting/test_reporter.py	99.0% <100.0%> (+<0.1%)	:arrow_up:
ixmp/tests/test_integration.py	100.0% <100.0%> (ø)

[khaeru]

@OFR-IIASA fyi, this is now merged.