[net] specify parameter name in parameterized test suite

* this renders as "HTTPTransport/HTTPTransport.ValidFormData_Gzip/http"
  instead of the default "HTTPTransport/HTTPTransport.ValidFormData_Gzip/0"
* switch the parameter type from a base::FilePath::StringType to a string

Change-Id: I19743966406f92176c566827d74a79aef5a87bb5
Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/1900324
Reviewed-by: Scott Graham <scottmg@chromium.org>
Commit-Queue: Francois Rousseau <frousseau@google.com>