Add ability to disable building tests

[kasper93]

As title says, would be nice to disable building tests, if not required.

option('tests', type: 'feature', value: 'enabled', yield: true, description: 'Enable or disable tests')

or

option('tests', type: 'boolean', value: false, description: 'Enable or disable tests')

and simply skip subdir('testbed') if disabled.

Thanks