runtime: do not use a relative import in testdata

Relative imports do not work in module mode. Use a fully-qualified
import path instead.

Updates #30228

Change-Id: I0a42ffa521a7b513395e7e1788022d24cbb1f31a
Reviewed-on: https://go-review.googlesource.com/c/go/+/165817
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/runtime/testdata/testprogcgo/dll_windows.go

@@ -12,7 +12,7 @@ DWORD getthread() {
 }
 */
 import "C"
-import "./windows"
+import "runtime/testdata/testprogcgo/windows"
 
 func init() {
 	register("CgoDLLImportsMain", CgoDLLImportsMain)