Complete `wasm_runtime.so` unit tests

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Is your feature request related to a problem? Please describe.

The wasm_runtime.so Rust library is at the core of mod_wasm.
https://github.com/vmware-labs/mod_wasm/tree/main/wasm_runtime/src

Thus, it requires minimal unit tests to harden its stability.

Describe the solution you'd like

Originally, minimal unit tests were included for the low-level const_c_char_to_str() at ffi_utils.rs.
The unit tests coverage for this library should be increased in all files.

• ffi_utils.rs
• c_api.rs
• config.rs
• module.rs
• wasi_ctx.rs
• wasm_engine.rs
• execution_ctx.rs

Describe alternatives you've considered

No response

Additional context

No response