Adds test to ensure that numericSeparator plugin does not alter "_" handling in Identifier

[rwaldron]

Signed-off-by: Rick Waldron waldron.rick@gmail.com

Q	A
Bug fix?	no
Breaking change?	no
New feature?	no
Deprecations?	no
Spec compliancy?	yes
Tests added/pass?	yes
Fixed tickets	comma-separated list of tickets fixed by the PR, if any
License	MIT

Per response to @jridgewell @hzoo @diervo

[codecov]

Codecov Report

Merging #548 into master will decrease coverage by 1.33%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #548      +/-   ##
==========================================
- Coverage   98.14%   96.81%   -1.34%     
==========================================
  Files          22       22              
  Lines        3607     3607              
  Branches     1007     1007              
==========================================
- Hits         3540     3492      -48     
- Misses         24       58      +34     
- Partials       43       57      +14

Flag	Coverage Δ
#babel	?
#babylon	96.81% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
src/parser/comments.js	87.87% <0%> (-6.07%)	⬇️
src/parser/expression.js	93.39% <0%> (-3.46%)	⬇️
src/tokenizer/state.js	97.14% <0%> (-2.86%)	⬇️
src/plugins/flow.js	97.35% <0%> (-1.04%)	⬇️
src/tokenizer/index.js	97.41% <0%> (-1%)	⬇️
src/parser/statement.js	98.23% <0%> (-0.89%)	⬇️

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