[JENKINS-61711] Fix validateExecutable to allow Git plugin global config...

[JENKINS-61711] Fix validateExecutable to allow Git plugin global config correctly report git executable in PATH (#4653)

* Fix git not in path for Git plugin global config
Git Client Plugin global configuration allows addition of git implementations using GitTool
The field Path was not able to identify git executable. The fix involves changing prefix with
File.seperator instead of File.pathSeperator. This allows Jenkins to find the git exec at the
already existing path.

* Tests added to validate JENKINS-61711 fix

* Improve hamcrest comparison in FormValidationTest

Co-Authored-By: Mark Waite <mark.earl.waite@gmail.com>

* Import matchers for `not`

Co-authored-by: Mark Waite <mark.earl.waite@gmail.com>