test(client): added clientMode option test

[knagaitsev]

• This is a bugfix
• This is a feature
• This is a code refactor
• This is a test update
• This is a docs update
• This is a metadata update

For Bugs and Features; did you add new tests?

Yes

Motivation / Use-Case

I should have added this earlier, but this simply confirms that the server is passing stuff into the clientMode helper correctly, and is throwing with bad clientMode.

Breaking Changes

None

Additional Info

I will add this mocking method to check serverMode helper usage if it looks fine.

[codecov]

Codecov Report

Merging #2078 into master will increase coverage by 0.16%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2078      +/-   ##
==========================================
+ Coverage   92.75%   92.92%   +0.16%     
==========================================
  Files          32       32              
  Lines        1201     1201              
  Branches      335      335              
==========================================
+ Hits         1114     1116       +2     
+ Misses         83       81       -2     
  Partials        4        4

Impacted Files	Coverage Δ
client-src/clients/SockJSClient.js	100% <0%> (+13.33%)	⬆️

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[alexander-akait]

/cc @hiroppy