Unable to parameterize test with the `-P` option when the parameter values contain `=`

[vkarak]

This is due to this:

reframe/reframe/frontend/cli.py

Line 1368 in 4814784

var, values_spec = param_spec.split('=')

We should split only once. This is related to #3264.

[jack-morrison]

Max split of 1

param_spec.split('=', maxsplit=1) (or similar)