Use resolved executable in KProcessRunner::fromExecutable

The other two code paths do this implicitely via KProcess::setShellCommand and DesktopExecParser::resultingArguments. This does not matter when the current process is starting the new process but is needed when the new process is going to be started by systemd because it might have a different PATH than the current process. A new test is added that tests the case of a binary only being in the PATH of the current process.