cmd/go: fix TestFFLAGS for Fortran compilers that accept unknown options

The test assumed that passing an unknown option to the Fortran
compiler would cause the compiler to fail. Unfortunately it appears
that some succeed. It's irrelevant to the actual test, which is
verifying that the flag was indeed passed.

Fixes #19080.

Change-Id: Ib9e89447a2104e4742f4b98938373fc2522772aa
Reviewed-on: https://go-review.googlesource.com/37658
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/cmd/go/go_test.go

@@ -3795,6 +3795,11 @@ func TestFFLAGS(t *testing.T) {
 	`)
 	tg.tempFile("p/src/p/a.f", `! comment`)
 	tg.setenv("GOPATH", tg.path("p"))
-	tg.runFail("build", "-x", "p")
+
+	// This should normally fail because we are passing an unknown flag,
+	// but issue #19080 points to Fortran compilers that succeed anyhow.
+	// To work either way we call doRun directly rather than run or runFail.
+	tg.doRun([]string{"build", "-x", "p"})
+
 	tg.grepStderr("no-such-fortran-flag", `missing expected "-no-such-fortran-flag"`)
 }