Add test for invalid UTF8

[hp77-creator]

Fixes #307

I have added a test which will check for InvalidUtf8 when the nodeId is not proper.

I tried adding separate test for InvalidUtf8 error code but Rust doesn't allow setting invalid utf-8 characters in a string. Hence have created a separate test.

[hp77-creator]

@vatsa287 please check once.

[vatsa287]

@hp77-creator Please do rustfmt <file> for the format test to pass.

[hp77-creator]

@hp77-creator Please do rustfmt <file> for the format test to pass.

Did it. Although I ran the following commands before submitting the PR:

• Build - cargo build --release

• Clippy - `cargo clippy --all --no-deps --all-targets --features=runtime-benchmarks -- -D warnings

both worked fine, so I submitted it, I guess, rustfmt should also be added in the checklist in README.md

[hp77-creator]

@vatsa287 can you check this?