chore: Run tests with Node 15

[nolanmar511]

This change will also result in Node 15 prebuilt binaries being released.

Two changes were made to testing as part of allowing tests with Node 15 to pass:

• npm run clean is called when the test script exits. This ensures there are no stale binaries or javascript generated from typescript when the next test starts.
• nvm is installed in /bin rather than /root. This is probably where is always should have been installed and eliminated permission issues during npm install. (I think the permission issues observed were related to npm install using a non-root user to run installation scripts combined with "/root" being the working directory).

[nolanmar511]

Node 15 system test fails with:

+ timeout 60 npm install --quiet --nodedir=/root/.nvm/versions/node/v15.2.1
/usr/bin/env: 'node': Permission denied
npm ERR! code 126
npm ERR! path /src
npm ERR! command failed
npm ERR! command sh -c node-pre-gyp install --fallback-to-build

This is the same failure we see with V8 canary test.

[codecov-io]

Codecov Report

Merging #146 (4187200) into master (01e0ac1) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #146   +/-   ##
=======================================
  Coverage   42.21%   42.21%           
=======================================
  Files          14       14           
  Lines        2061     2061           
  Branches       42       42           
=======================================
  Hits          870      870           
  Misses       1173     1173           
  Partials       18       18           

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