Test the AWS XRAY propagator fields

[MrAlias]

Test the functionality provided:

opentelemetry-go-contrib/propagators/aws/xray/propagator.go

Lines 172 to 175 in 836e94b

	// Fields returns list of fields used by HTTPTextFormat.
	func (xray Propagator) Fields() []string {
	return []string{traceHeaderKey}
	}

The test should verify the singular response of this method.

[Oloruntobi1]

@MrAlias can this be assigned to me?