end-to-end tests for "rnx-test" command

[afoxman]

Validate "rnx-test" on cross-platform test suite (use test-app package). Also, for non-CLI users, validate that the same package passes using Jest and the rnx-kit Jest preset.

In both scenarios, verify that snapshots work and are per-platform.

[kelset]

update: all that's missing is the snapshot part at this point in time