runners: tests: don't patch BuildConfiguration

Instead of mocking out the BuildConfiguration class, just create its
input file and let the real class do the work.

This in turn exposes a bug in the way the board name is being pulled
out of the BuildConfiguration, which we fix to keep the tests passing.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
4 files changed