net/http: make use of testing.Errorf instead of testing.Fatalf in non-test function goroutines
net/http: make use of testing.Errorf instead of testing.Fatalf in non-test function goroutines

See testing.FailNow for further information.
Also avoid nil pointer derefernce in TestTransportMaxPerHostIdleConns.

LGTM=dave
R=golang-codereviews, dave
CC=golang-codereviews
https://codereview.appspot.com/76470043

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