test(typescript-estree): separate snapshots for ast tests

[bradzacher]

Closes #1491
Fixes #1142

Following in the steps of what I did for scope manager and the exploratory work we've done in the past, this uses jest-specific-snapshot to create a snapshot per fixture instead of a mega snapshot.

This should make it easier to review AST changes and should be easier to inspect and maintain.

[typescript-eslint]

Thanks for the PR, @bradzacher!

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Thanks again!

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[codecov]

Codecov Report

Merging #2290 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2290      +/-   ##
==========================================
- Coverage   93.12%   93.11%   -0.02%     
==========================================
  Files         283      283              
  Lines        9057     9057              
  Branches     2482     2482              
==========================================
- Hits         8434     8433       -1     
  Misses        301      301              
- Partials      322      323       +1     

Flag	Coverage Δ
#unittest	93.11% <ø> (-0.02%)	⬇️

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

Impacted Files	Coverage Δ
...ript-estree/src/create-program/createSourceFile.ts	87.50% <0.00%> (-12.50%)	⬇️