Support verifying no-std support through serde_json_test

Tested with:

    cargo check \
        --manifest-path tests/crate/Cargo.toml \
        --target aarch64-unknown-none \
        --no-default-features \
        --features alloc

which fails before this commit and succeeds after.

Before this commit:

        Checking serde_json_test v0.0.0
    error[E0463]: can't find crate for `std`
      |
      = note: the `aarch64-unknown-none` target may not be installed

    error: aborting due to previous error

tests/crate/test.rs

@@ -1 +1,3 @@
+#![no_std]
+
 pub use serde_json::*;