Fix handling of escaped string in tokenizer + PTVS launch hardening

[MikhailArkhipov]

Fixes #1364

• do not consider single-quoted string multiline even if it is not terminated

Harden language sever launch

• handle case when assemblies are completely missing better

Match PTVS changes in microsoft/PTVS#4024

• pass test flag differently and pass new async startup flags

This pull request:

• Has a title summarizes what is changing
• Has unit tests & code coverage is not adversely affected (within reason)
• Works on all actively maintained versions of Python (e.g. Python 2.7 & the latest Python 3 release)
• Works on Windows 10, macOS, and Linux (e.g. considered file system case-sensitivity)

[DonJayamanne]

this.deferred.reject(error)

[codecov]

Codecov Report

Merging #1377 into master will decrease coverage by 0.3%.
The diff coverage is 22.22%.

@@            Coverage Diff             @@
##           master    #1377      +/-   ##
==========================================
- Coverage   71.45%   71.14%   -0.31%     
==========================================
  Files         269      269              
  Lines       12403    12416      +13     
  Branches     2199     2199              
==========================================
- Hits         8862     8833      -29     
- Misses       3411     3452      +41     
- Partials      130      131       +1

Impacted Files	Coverage Δ
src/client/language/tokenizer.ts	96.8% <100%> (+0.02%)	⬆️
src/client/activation/analysis.ts	15.45% <12.5%> (-0.71%)	⬇️
src/client/linters/errorHandlers/notInstalled.ts	33.33% <0%> (-61.12%)	⬇️
src/client/linters/errorHandlers/errorHandler.ts	77.77% <0%> (-22.23%)	⬇️
src/client/common/logger.ts	53.33% <0%> (-20.01%)	⬇️
src/client/common/application/applicationShell.ts	23.07% <0%> (-7.7%)	⬇️
src/client/common/installer/productInstaller.ts	60.31% <0%> (-4.24%)	⬇️
src/client/linters/baseLinter.ts	87.5% <0%> (-4.17%)	⬇️
...reter/locators/services/cacheableLocatorService.ts	91.83% <0%> (-4.09%)	⬇️
...rc/client/debugger/PythonProcessCallbackHandler.ts	52.96% <0%> (-0.66%)	⬇️
... and 1 more

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