parallel-checkout: add tests for basic operations

parallel-checkout: add tests for basic operations

Add tests to populate the working tree during clone and checkout using
sequential and parallel mode, to confirm that they produce identical
results. Also test basic checkout mechanics, such as checking for
symlinks in the leading directories and the abidance to --force.

Note: some helper functions are added to a common lib file which is only
included by t2080 for now. But they will also be used by other
parallel-checkout tests in the following patches.

Co-authored-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>