Add ability to get browser mob proxy instance.

[rosolko]

• Simplify and extend proxy test.
• Exclude provided by webdrivermanager logback-classic module.
• Use new testImplementation scope instead of old testCompile.

Proposed changes

Describe the big picture of your changes here to communicate to the maintainers why we should accept this pull request.
If it fixes a bug or resolves a feature request, be sure to link to that issue.

Checklist

• Checkstyle and unit tests pass locally with my changes by running gradle check chrome htmlunit command
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[coveralls]

Coverage increased (+0.04%) to 66.975% when pulling 607d160 on rosolko:openProxy into 35c26b6 on codeborne:master.

[codecov-io]

Codecov Report

Merging #731 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##             master    #731      +/-   ##
===========================================
+ Coverage     63.65%   63.7%   +0.04%     
- Complexity      877     878       +1     
===========================================
  Files           154     154              
  Lines          3024    3025       +1     
  Branches        298     298              
===========================================
+ Hits           1925    1927       +2     
+ Misses         1000     999       -1     
  Partials         99      99

Impacted Files	Coverage Δ	Complexity Δ
.../codeborne/selenide/proxy/SelenideProxyServer.java	87.8% <100%> (+0.3%)	11 <1> (+1)	⬆️
...e/selenide/impl/WebDriverThreadLocalContainer.java	81.61% <0%> (+0.73%)	30% <0%> (ø)	⬇️

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