Fix two failing tests

[danielhuppmann]

Description of PR

This PR fixes two failing tests:

• for some reason, the UNFCCC inventory changes from time to time (marginally but enough to trip the tests)
• change the implementation of the IIASA API tests

The IIASA "integration-test" database instance was now set to public, the configurations were defined incorrectly as part of the migration to a new services-manager (see here for more info).

[codecov]

Codecov Report

Merging #675 (a7b17f0) into main (516da60) will increase coverage by 0.0%.
The diff coverage is 91.6%.

@@          Coverage Diff          @@
##            main    #675   +/-   ##
=====================================
  Coverage   94.6%   94.7%           
=====================================
  Files         59      59           
  Lines       5776    5768    -8     
=====================================
- Hits        5465    5463    -2     
+ Misses       311     305    -6     

Impacted Files	Coverage Δ
tests/test_unfccc.py	100.0% <ø> (ø)
pyam/iiasa.py	85.8% <75.0%> (-0.1%)	⬇️
tests/test_iiasa.py	96.8% <100.0%> (+4.2%)	⬆️

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