Fix a bug with DataQualityFlag subtraction & revise a few tests

[asouthgate]

This fixes a bug in DataQualityFlag subtraction (https://github.com/gwpy/gwpy/issues/1700) documentation and improves the test for it.

This also changes a few other test assertions in DataQualityDict which I believe were not testing correct elements.

E.g.

    def __isub__(self, other):
        for key, value in other.items():
            if key in self:
                self[key] -= value
        return self

Subtraction is based on keys. However, the tests were testing on order, which was not the same thing.

[codecov]

Codecov Report

Merging #1703 (03804e9) into main (bc48f2c) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main   #1703   +/-   ##
=====================================
  Coverage   94.3%   94.3%           
=====================================
  Files        239     239           
  Lines      18560   18575   +15     
=====================================
+ Hits       17517   17533   +16     
+ Misses      1043    1042    -1     

Flag	Coverage Δ
Conda	94.1% <100.0%> (+<0.1%)	⬆️
Linux	94.3% <100.0%> (+<0.1%)	⬆️
Windows	84.7% <100.0%> (+<0.1%)	⬆️
macOS	93.6% <100.0%> (+<0.1%)	⬆️
python3.10	94.1% <100.0%> (+0.5%)	⬆️
python3.11	94.1% <100.0%> (+0.4%)	⬆️
python3.8	94.3% <100.0%> (+<0.1%)	⬆️
python3.9	94.1% <100.0%> (+<0.1%)	⬆️

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Files	Coverage Δ
gwpy/segments/tests/test_flag.py	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

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