test: skip installation test if GOOGLE_CLOUD_TESTS_IN_VPCSC is passed

[jkwlui]

Split out install-test as a separate npm run-script instead of bundling
with system-test.

This helps unblock VPCSC testing since it can't fetch external
dependencies with npm install when installation test does npm pack

[JustinBeckwith]

Instead of this - could we do the thing you did in storage where we looked for an env var, and did an explicit skip? I don't want the kokoro configs straying if we can at all avoid it.

[jkwlui]

I changed it to use the environment variable to determine whether or not to skip install-test, but I still kept it split out

[codecov]

Codecov Report

Merging #345 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #345   +/-   ##
=======================================
  Coverage   99.44%   99.44%           
=======================================
  Files           4        4           
  Lines         544      544           
  Branches       75       75           
=======================================
  Hits          541      541           
  Misses          2        2           
  Partials        1        1

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