tests: remove default firmware config for dfx-mgr in binary tests

When running the binary tests (non-snap), dfx-mgr is installed via apt as a dependency. The dfx-mgr package installs a default firmware configuration that could potentially conflict with fpgad behaviour, particularly after a system reboot.

**Context:** Identified in PR https://github.com/canonical/fpgad/pull/168 at [.github/testflinger-assets/test_binary_device_script.sh line 7](https://github.com/canonical/fpgad/blob/artie/dfx-mgr-tests/.github/testflinger-assets/test_binary_device_script.sh#L7), where dfx-mgr is installed via apt.

**Proposed Fix:** Remove or override the default firmware config for dfx-mgr during binary test setup to avoid conflicts with fpgad after a reboot.

Open .github/testflinger-assets/test_binary_device_script.sh at line 7 and inspect how dfx-mgr is installed via apt. Identify the package's default firmware configuration and remove or override it during binary test setup. Run the non-snap binary tests and verify the configuration cannot conflict with fpgad after a reboot.

Written by the indexing model from the issue text.