Fix ruby version check in plugin test

[adamruzicka]

Before

$ if ["${ruby}" = '2.7']; then echo yay; fi
bash: [2.7: command not found

After

$ if [ "${ruby}" = '2.7' ]; then echo yay; fi
yay

Fixes: f164a3f

[adamruzicka]

Hopefully should address this kind of failure https://ci.theforeman.org/job/foreman_openscap-pull-request/531/label=fast,ruby=2.7/console

[ekohl]

I was debating this when I reviewed it. @pcreech guess I should have commented after all ;)