test: Added Ubuntu 20.04 (Focal Fossa) Installation Test Case

[aymanzay]

Ubuntu 20.04 cannot add the same ppa repositories as 16/18.04, so apt install build-essentials is used to install g++-7.

Issue being fixed or feature implemented

• Missing test builds for multiple Ubuntu versions; now running Xenial and Focal build stages.
  Related issue: Trouble with the install scripts step on Windows (WSL) using conda #179

What was done?

• Added new test case for Ubuntu 20.04 with Xenial-based commands are replaced with apt install build-essentials.

How Has This Been Tested?

• Locally - Docker and Travis

Breaking Changes

None

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation

[versey-sherry]

@aymanzay I edited the PR description for you.

[versey-sherry]

and for the earlier versions of linux users should run

sudo apt-get update
sudo apt-get upgrade
sudo apt install build-essential
sudo apt -y install g++-7
export CC="$(which gcc-7)"
export CXX="$(which g++-7)"

gcc-7 g++7 part could also just be gcc

@versey-sherry change the docs for Windows and Linux.

[versey-sherry]

@versey-sherry change docs before merging.