Add TestParallelHandshake test case

[Danielius1922]

Handshake are synchronized by a mutex, but there is a panicing scenario when the first handshake fails.

Description

Reference issue

Fixes #...

[Danielius1922]

@Sean-Der: The problematic scenario seems to go like this:

Goroutine A: Gets handshake mutex
Goroutine B: Waits on handshake mutex
Goroutine A: Handshake fails
             close(c.decrypted)
Goroutine B: Gets handshake mutex
             Handshake succeeds or fails
             close(c.decrypted)
             panic - closing of a closed channel

If the first handshake succeeds the issue does not occur.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.13%. Comparing base (f3e8a9e) to head (38b2704).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #666      +/-   ##
==========================================
+ Coverage   80.05%   80.13%   +0.07%     
==========================================
  Files         101      101              
  Lines        5355     5355              
==========================================
+ Hits         4287     4291       +4     
+ Misses        697      694       -3     
+ Partials      371      370       -1     

Flag	Coverage Δ
go	80.16% <100.00%> (+0.07%)	⬆️
wasm	64.30% <100.00%> (+0.03%)	⬆️

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