fix(Justfile): fix test-threads to max 2

- The problem: When running `cargo test --all-
features`, the backends (electrum, rpc, esplora
from TestEnv) run in parallel at the number of
cpus available and exhaust the processes/ports
available, thereby resulting in os error 35.

This fix limits the number of threads that are
available for the tests to run in parallel.

Author: tvpeter

Justfile

@@ -28,7 +28,7 @@ test:
 pre-push:
     cargo test --features default
     cargo test --no-default-features
-    cargo test --all-features
+    cargo test --all-features -- --test-threads=2
     cargo clippy --no-default-features --all-targets -- -D warnings
     cargo clippy --all-features --all-targets -- -D warnings
     cargo fmt --all -- --check