reftest: untie lock test from OPAMEDITOR hack, and fix lock VCS remote detection

[rjbou]

The change in lock test is needed for #6319.
Queued on

• lock: add '--keep-local' argument to add local pins to 'pin-depends:' field #6411

edit:

• untied from Fix opam file lookup when a pin is done with a local directory and a branch is specified  #6409
• No more in the PR: While changing the test, i found the bug with git remote get-url, which is specific to opam lock handling, extracted in git: handle exit code 2 for git remote url retrieval #6422

[kit-ty-kate]

Thanks a lot!