cmd/cgo/internal/testfortran: relax test output

cmd/cgo/internal/testfortran: relax test output

Some new linker may emit warning message to standard error, causing
false positive in test result.

Fixing this by testing only stdout output.

Fixes #63588

Change-Id: I272048c41dc1c316f44af2dfc903bb03383baea3
Reviewed-on: https://go-review.googlesource.com/c/go/+/535975
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>