Set testinfra builder scenario to parallel
Set testinfra builder scenario to parallel

This is really a pytest option, `-n numprocesses`. Setting this to auto mode is
supposed to use CPU detection for ideal number of workers.

garrettr/securedrop,micahflee/securedrop,garrettr/securedrop,heartsucker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,garrettr/securedrop,ehartsuyker/securedrop,conorsch/securedrop,micahflee/securedrop,ehartsuyker/securedrop,conorsch/securedrop,heartsucker/securedrop,micahflee/securedrop,heartsucker/securedrop,heartsucker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,conorsch/securedrop,micahflee/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,garrettr/securedrop