Pass a command line flag to downstream tests

[jishnub]

The command line flag --downstream_integration_test that is passed to the downstream test will allow us to selectively skip tests that might not be necessary. In particular, the Aqua test for stale dependencies is unnecessary in downstream tests if we want to test package extensions.

We also reinstate the test against InfiniteLinearAlgebra in this PR, which was commented out.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.55%. Comparing base (a62bd8e) to head (c041123).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #435   +/-   ##
=======================================
  Coverage   93.55%   93.55%           
=======================================
  Files          18       18           
  Lines        1644     1644           
=======================================
  Hits         1538     1538           
  Misses        106      106           

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