Fix build by disabling un-pinned babel test

[alangpierce]

It looks like a number of fixture tests were depending on core-js-compat in an
un-pinned way. Rather than trying to stay constantly up-to-date, this PR just
disables those tests, since there are plenty of other useful and reliable Babel
tests.

[codecov-io]

Codecov Report

Merging #480 into master will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #480      +/-   ##
==========================================
- Coverage   81.67%   81.59%   -0.08%     
==========================================
  Files          52       52              
  Lines        5429     5200     -229     
  Branches     1207     1207              
==========================================
- Hits         4434     4243     -191     
+ Misses        702      667      -35     
+ Partials      293      290       -3

Impacted Files	Coverage Δ
src/Options-gen-types.ts	50% <0%> (-50%)	⬇️
src/util/getDeclarationInfo.ts	80% <0%> (-20%)	⬇️
src/util/elideImportEquals.ts	81.81% <0%> (-18.19%)	⬇️
src/parser/index.ts	77.77% <0%> (-7.94%)	⬇️
src/computeSourceMap.ts	83.33% <0%> (-5.56%)	⬇️
src/util/getNonTypeIdentifiers.ts	94.44% <0%> (-5.56%)	⬇️
src/parser/plugins/types.ts	91.66% <0%> (-3.34%)	⬇️
src/parser/tokenizer/readWord.ts	82.35% <0%> (-3.02%)	⬇️
src/util/shouldElideDefaultExport.ts	75% <0%> (-2.78%)	⬇️
src/parser/traverser/util.ts	86.2% <0%> (-2.17%)	⬇️
... and 17 more

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