Encapsulate config.rb into TestSuiteConfig class

Similar to #9030, it’s potentially problematic to define stuff in the global context except classes and modules. We could fix this pretty easily by implementing a TestSuiteConfig class that inherits from Hash, and does the YAML stuff in initialize and then do $config = TestSuiteConfig.new in the global context (which is fine). That wouldn’t require any change to code using $config anywhere else.

We probably should move the constants into this class too, which would require e.g. TAILS_ISO to be changed to TestSuiteConfig.TAILS_ISO which actually looks like a good thing since it’s easier to understand that the constant is part of the test suite’s configuration.

Parent Task: #10237

Original created by @anonym on 9040 (Redmine)