[dvsim] Allow multiple `en_build_mode` values for a single test

This stems from an offline discussion with @sriyerg.

Allowing dvsim to handle multiple build_modes being specified on a per-test basis could potentially allow for greater ease of setting up testlists for DUTs with multiple parameter configurations.

Example use-case is the KMAC IP, where we want to test both the masked and unmasked versions (controlled by a compile-time parameter). Since only one build_mode is supported per test the current approach is to create two Hjson config files (and testlists) for each of the parameter sets.