cmd/go: remove "x" in TestImportMain

Interestingly, this only became a problem when CL 32850 marked
TestImportMain as parallel.  Before that, "x" was overwritten and remove
in a later test, TestGoBuildOutput.  The latter test is not marked as
parallel, so now it is run first.  It is rather fragile for two tests to
compete over the same filename, but this change is correct regardless.

Change-Id: I1db7929c0bc20a2fd0cc6a02999bef2dca9e0cc0
Reviewed-on: https://go-review.googlesource.com/33092
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/go_test.go

@@ -2575,6 +2575,7 @@ func TestImportMain(t *testing.T) {
 		func TestFoo(t *testing.T) {}
 	`)
 	tg.setenv("GOPATH", tg.path("."))
+	tg.creatingTemp("x")
 	tg.run("build", "x")
 	tg.run("test", "x")