[1.8.x] BATS bash tests for build scripts + various other improvements and fixes

[NorseGaud]

Change Description

• BATS-CORE is being used for bash unit testing: This will allow us to ensure bash scripts function as they should for users. It's perfect for regression testing and consistency. (see tests/bash-bats/README.md)
• CICD Pipeline/steps run as the user cicd (instead of root)
  • Improvements to log output in buildkite showing a collapsed tab for each and every command the scripts run.
  • Auto retry failed step on -1 (Agent Lost)
• Users are prompted if 'which' is missing (we will install it for them if they want): linux build scripts "which/c++: command not found" #7232
• All prompts are now properly wrapped to be user friendly: scripts/eosio_build.sh is unfriendly when asking "Do you wish to update repositories"? #6931
• New eosio path: ~/eosio/$VersionFromCmakeFile/ to store dependencies and eosio: scripts/eosio_build.sh is unfriendly when asking "Do you wish to update repositories"? #6931
• Script filename extensions are now .bash, indicating that bash is required to properly execute.
• Scripts now use #!/usr/bin/env to pull bash if it's installed into nonstandard /bin/ location
• Better uninstaller script, removing old "full_uninstaller" (readme updated)
• Build scripts support DRYRUN=true (doesn't execute anything, but shows you what it would do) and VERBOSE=true (shows the exact commands executed throughout script) for better troubleshooting.
• Scripts use set -ieo pipefail so they fail properly when a command fails and also show colors for users executing the script (colors not supported on mac yet).
• Using *_deps file for each distro
  • Package dependency files include flexibility by allowing us to specify what command is used to check if it exists or not (dpkg -s/rpm -qa)
  • no more extra space at end of deps file! (yay)
• Using helpers/functions for modularity, avoiding having to repeat ourselves in each eosio_build_* script
• Environment variables have been moved into a separate file for better flexibility
  • Test scripts can pull in the same variables
  • comments added for better comprehension

Compiler support

_You can see almost all of these tests passing in https://buildkite.com/EOSIO/eosio-base-images/builds/170

OS	-P	existing compiler	existing c++/clang < required version	no existing compiler
Darwin	✅	✅	✅ Will prompt for pin/install clang8	✔️ (Hasn't been tested)
Centos 7	✅	✅ devtoolset-7	✔️ (Hasn't been tested)	✅ Will prompt for install of build-essentials (fails: see below)
AmazonLinux 2	✅	✅ gcc-c++	✔️ (Hasn't been tested)	✅ exits and notifies user
Ubuntu 16.04	✅	✔️ (Hasn't been tested)	✅ Will prompt for pin/install clang8	✅ exits and notifies user
Ubuntu 18.04	✅	✔️ (Hasn't been tested)	✅ Will prompt for pin/install clang8	✅ Will prompt for install of build-essentials

List of issues

1. scripts/eosio_build.sh is unfriendly when asking "Do you wish to update repositories"? #6931 : Prompts are now humane + install into version specific folder
2. centos build script has no sudo; implicitly encourages build as root #7120 : Centos 7 will now use sudo for yum commands if $(whoami) = root
3. build script boost detection inconsistent and causes grief via not finding boost #7262 : New directory structure should fix this
4. build script on clean install has compiler detection error #7260 : New logic should prevent this from being a problem
5. linux build scripts "which/c++: command not found" #7232 : User prompts if missing which or c++ (and not using -P).

Consensus Changes

• Consensus Changes

API Changes

• API Changes

Documentation Additions

• Documentation Additions