Some fields of `RegressionTest` are too restrictive

[vkarak]

For example, the variables field. We now require it to be of type dict[str, str], but in fact any type of values could be used, since the str() is called internally. This would allow us to use the following syntax:

self.variables = {
    'OMP_NUM_THREADS': self.num_cpus_per_task
}

instead of the current one:

self.variables = {
    'OMP_NUM_THREADS': str(self.num_cpus_per_task)
}

Internal issue: https://madra.cscs.ch/scs/reframe/issues/562