Add test coverage

[cspotcode]

Some of these tests are silly, but they were easy to implement and bump the coverage up. I personally don't believe in coverage numbers out of context, but I know other people (incorrectly) do, so why not?

Blocked by #1103 which will be merged soon

[codecov]

Codecov Report

Merging #1109 into master will increase coverage by 3.19%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1109      +/-   ##
==========================================
+ Coverage   75.68%   78.87%   +3.19%     
==========================================
  Files           7        7              
  Lines         658      658              
  Branches      147      147              
==========================================
+ Hits          498      519      +21     
+ Misses        107       91      -16     
+ Partials       53       48       -5     

Flag	Coverage Δ
#node_10	75.42% <ø> (+3.02%)	⬆️
#node_12_15	75.80% <ø> (+3.02%)	⬆️
#node_12_16	75.80% <ø> (+3.02%)	⬆️
#node_13	78.57% <ø> (+3.19%)	⬆️
#node_14	78.57% <ø> (+3.19%)	⬆️
#typescript_2_7	78.41% <ø> (+3.19%)	⬆️
#typescript_latest	77.65% <ø> (+3.19%)	⬆️
#typescript_next	77.50% <ø> (+3.19%)	⬆️
#ubuntu	78.57% <ø> (+3.19%)	⬆️
#windows	78.72% <ø> (+3.19%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/index.ts	79.60% <0.00%> (+1.41%)	⬆️
src/bin.ts	76.30% <0.00%> (+6.35%)	⬆️
src/esm.ts	80.00% <0.00%> (+11.11%)	⬆️

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[coveralls]

Coverage increased (+4.2%) to 86.454% when pulling 98dc6f5 on ab/esm-loader-test-deferred-urls into e29ae04 on master.