webhook-http-proxy-test: print squid logs on failure and improve access log grep

[bwagner5]

Issue #, if available:
N/A

Description of changes:

• webhook-http-proxy-test
  • print squid logs on failure
  • improve access log grep to only look for the webhook proxy server address

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Merging #214 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #214   +/-   ##
=======================================
  Coverage   81.43%   81.43%           
=======================================
  Files          10       10           
  Lines         792      792           
=======================================
  Hits          645      645           
  Misses        131      131           
  Partials       16       16           

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[cjerad]

Nice work