chore(solc): fix flaky integration test

When running integration tests, the `cfg(test)` is not triggered, so we
added a `tests` feature in the code. However, we did not include it
in the actual features of the crate, so it was not getting triggered
when the `--all-features` tests were running.

ethers-solc/Cargo.toml

@@ -39,3 +39,4 @@ tempdir = "0.3.7"
 [features]
 async = ["tokio", "futures-util"]
 full = ["async", "svm"]
+tests = []