when kioslave5 couldn't be found in libexec-ish locations try $PATH

Summary:
on windows we install the binary to bin/ where software that uses kio
can find it through applicationDirPath() ordinarily. at test-time however
applicationDirPath() of the test != finalPath, so the only way a test
can possibly find the helper is through $PATH.

this should fix the drkonqi connection test on win32 on build.kde.org

Reviewers: dfaure, apol

Reviewed By: apol

Subscribers: anthonyfieroni, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25308