meson.build: Set RUST_BACKTRACE for all tests

meson.build: Set RUST_BACKTRACE for all tests

We want to capture potential Rust backtraces on panics in our test
logs, which isn't Rust's default behaviour.  Set RUST_BACKTRACE=1 in
the add_test_setup environments, so that all our tests get run with
this environment variable set.

This makes the setting of that variable in the gitlab CI template
redundant, so we can remove it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20250310102950.3752908-1-peter.maydell@linaro.org