feat(tests): option to use custom emulator version in local e2e tests

[grdddj]

Addressing trezor/trezor-user-env#49:

• ability to run Suite e2e tests against a locally-built version of the emulator
• it is certainly open for improvements, it is just a running prototype

Current usage (for @matejcik):

• setting up Suite locally (this and this guide)
• building emulator version locally
• being in root of the Suite repo, running the testing script and giving it the locations of the custom emulator(s) - docker/docker-suite-test.sh -o /tmp/1-custom -t /tmp/2-custom

[grdddj]

Considerations:

• copying the custom emulator under X-master (X being 1 or 2) might not be the best, because it will overwrite the current X-master version, but it is by far the easiest solution (as Suite e2e tests are already choosing X-master versions for majority of tests)
  • as made possible in Grdddj/replacing old firmwares in e2e tests #4079, we should be able to set the emulator version just by setting environment variable in the Cypress container
  • meaning we can decide to name this custom emulator X-custom and set the env variable in docker-compose.yaml to CYPRESS_emuVersionTX=X-custom - it would just be more logic
• not sure if forwarding all the arguments from one script to the other and spawning the copying script on the background with sleep are good approaches