Fixed .BuildTestLib for covr

[jayqi]

covr has been broken with pkgnet for a while, since #108 was merged.

This has caused our codecov to silently fail in in the Travis "after_success" build steps.

The same error was seen when trying to run covr::package_coverage locally (or by trying to run FunctionReporter with calculate_test_coverage).

This was due to the way covr installs the package into a temporary directory before it does all of its stuff. The directory structure in the covr case wasn't included in the list of directory sanitation in .BuildTestLib.

Additionally, because we had to install the test packages from an installed copy of pkgnet source rather than the raw source, the test packages had all been moved out of inst up into the root package directory, so the path to the test package source had to be adjusted.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@71c6163). Click here to learn what that means.
The diff coverage is 87.5%.

@@            Coverage Diff            @@
##             master     #144   +/-   ##
=========================================
  Coverage          ?   86.62%           
=========================================
  Files             ?       10           
  Lines             ?      897           
  Branches          ?        0           
=========================================
  Hits              ?      777           
  Misses            ?      120           
  Partials          ?        0

Impacted Files	Coverage Δ
R/testing_utils.R	77.14% <87.5%> (ø)

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[jameslamb]

LGTM! Well done.