aconfig: re-enable development with cargo

The integration tests introduced in 906405f5 depend on libraries
auto-generated by running aconfig. This works fine for the Android build
system, but breaks `cargo t`.

Disable the tests when using `cargo t` for local development. Cargo can
be used for rapid local development, including running unit tests;
integration tests should be run using atest.

Bug: 298904077
Test: cargo t
Test: atest aconfig.test_mode.test.rust aconfig.prod_mode.test.rust
Change-Id: Ifcd374fa6efdc121cbfd4c4ac4fb43cf9cacc192