Extend parse tests with more use cases

[pellared]

https://github.com/w3c/baggage/blob/main/test/test_baggage.py can have more test cases added. Some can be taken from https://github.com/open-telemetry/opentelemetry-go/blob/5449f083aa4af79313d5fe5fecd554af2cb7bbbc/baggage/baggage_test.go#L239.

This can help implementers of the baggage propagator to avoid some common issues. E.g.

• + value should be a parsed to + (included in Go test cases)
• %20 key should be parsed to %20 as keys are NOT percent encoded (NOT included in Go test cases)

CC @lachmatt @vishweshbankwar