100% Test coverage and misc cleanup

[schmidt-sebastian]

This PR brings us back to 100% test coverage (which dropped to an all time low of 99% with #137) and cleans up some test RegExp syntax since we don't enfore line length anymore in tests.

This also moves the Field Path tests to one consolidated place.

Note: The 100% coverage change is

- .update('foo', 'bar', {exists: true}, 'foo');  
+ .update('foo', 'bar', {exists: true});

[codecov]

Codecov Report

Merging #139 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #139      +/-   ##
========================================
+ Coverage   99.94%   100%   +0.05%     
========================================
  Files          12     12              
  Lines        1689   1688       -1     
========================================
  Hits         1688   1688              
+ Misses          1      0       -1

Impacted Files	Coverage Δ
src/write-batch.js	100% <ø> (ø)	⬆️
src/reference.js	100% <ø> (ø)	⬆️
src/document.js	100% <100%> (+0.32%)	⬆️

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