Use browser's path for br-resume coookie

[mc2]

For multiple files in items, cookie paths written by code get broken and don't match with browser.
Use what the browser sees as the path, trimming any /page/... or /mode/...
Note: Required for multiple files in items petabox code

https://webarchive.jira.com/browse/WEBDEV-3542

Solution Summary:

• Implement regex trimming /path/ or /mode/ from location.pathname to use for cookie path
• Wrap in method for testability
• Refactor legacy test using .call() and breaking this
• Add unit tests of new wrapped regex

Testing
Should set/get cookies perfectly, independent of new petabox code to handle multiple files in items

[codecov]

Codecov Report

Merging #359 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #359      +/-   ##
==========================================
+ Coverage   60.61%   60.64%   +0.02%     
==========================================
  Files          32       32              
  Lines        3428     3430       +2     
  Branches      649      649              
==========================================
+ Hits         2078     2080       +2     
  Misses       1084     1084              
  Partials      266      266              

Impacted Files	Coverage Δ
src/js/plugins/plugin.resume.js	100.00% <100.00%> (ø)

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[iisa]

Code looks good!
Can merge when @mc2 confirms QA has passed.