Rename test repos to correspond to their purpose

[eugenepeh]

As per title, can modify the READMEof the test repo too to list their purpose if necessary.

The following is the current list of repositories and their usages, as at 31 March 2021:

Repository	Usages	Description
testrepo-Alpha	GitTestTemplate	Specialized repository for file and commit analysis, contains multiple branches
testrepo-Beta	Config parser tests	Generic repository for testing configuration parsing on a real repository with local configuration
testrepo-Charlie	Config parser tests	Generic repository for testing configuration parsing on a real repository without local configuration
testrepo-Delta	Config parser tests	Generic repository for testing configuration parsing on a real repository with local configuration (alternative)
testrepo-Empty	RepoClonerTest	Specialized repository for testing an empty repository
testrepo-MergeCommit	Unused	Unused
testrepo-Submodule	GitDiffTest	Specialized repository for serving as a Git submodule for testrepo-Alpha
testrepo-UncommonDefaultBranch	GitBranchTest	Specialized repository with an uncommon default branch (not master nor main)
testrepo-minimalStandaloneConfig	RepoConfigurationTest	Specialized repository with a minimal standalone configuration file

[dcshzj]

Original issue description updated with some information on the repositories, gathered from the usages in unit tests, as at d7e5d42.

testrepo-Alpha has 20+ branches each with their own specific purposes, which may be difficult to rename. -Beta, -Charlie and -Delta can be renamed, but their usages in testing configuration files need to be updated and consolidated first.

[ckcherry23]

It might be useful if information regarding these test repositories is added to the developer docs under a testing section, so that new contributors understand the intentions behind each test repo. (Or we can now use blurbs #2191)