Remove exec() call with invalid name on windows

Currently running unit tests prints the following message 3 times:
The command "command" is misspelt or could not be found.
Instead of trying this, we just skip this now.
Deduplicate function by moving it to the OC_Helper
Simplify the binary finding code in the movie preview class
Merge pull request #12303 from owncloud/windows-unknown-command-command

Deduplicate findBinaryPath() and do not try "command -v" on windows