Handle `cargo test -p wasm-pkg-client --no-default-features` lint warnings

[mkatychev]

$ cargo test -p wasm-pkg-client --no-default-features
   Compiling prost-build v0.12.6
   Compiling rustix v1.1.4
   Compiling pbjson-types v0.6.0
   Compiling warg-protobuf v0.9.2
   Compiling tempfile v3.27.0
   Compiling xattr v1.6.1
   Compiling astral-tokio-tar v0.5.6
   Compiling dialoguer v0.11.0
   Compiling testcontainers v0.25.2
   Compiling warg-transparency v0.9.2
   Compiling warg-protocol v0.9.2
   Compiling warg-api v0.9.2
   Compiling warg-client v0.9.2
   Compiling wasm-pkg-client v0.16.0 (/Users/mkatychev/Documents/rust/wasm-pkg-tools/crates/wasm-pkg-client)
warning: unused import: `futures_util::TryStreamExt`
 --> crates/wasm-pkg-client/tests/e2e.rs:1:5
  |
1 | use futures_util::TryStreamExt;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` (part of `#[warn(unused)]`) on by default

warning: constant `FIXTURE_WASM` is never used
 --> crates/wasm-pkg-client/tests/e2e.rs:4:7
  |
4 | const FIXTURE_WASM: &str = "./tests/testdata/binary_wit.wasm";
  |       ^^^^^^^^^^^^
  |
  = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default

warning: function `generate_self_signed_tls_fixture` is never used
   --> crates/wasm-pkg-client/tests/e2e.rs:158:4
    |
158 | fn generate_self_signed_tls_fixture() -> (String, String) {
    |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^