net/http: correct error messages for TestParseSetCookie

net/http: correct error messages for TestParseSetCookie

This change fixes typos in error messages, while here, also improves
'line' strings to indicate the errEqualNotFoundInCookie error.

Change-Id: I0ce6115c605844d2d86f337f208fd3b2d3774674
Reviewed-on: https://go-review.googlesource.com/c/go/+/579799
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Joedian Reid <joedian@google.com>
Auto-Submit: Damien Neil <dneil@google.com>