test: move empty header file in builddir, buildrundir to temp directory

Move the empty header file created by "builddir", "buildrundir"
directives to t.tempDir. The file was accidentally placed in the
same directory as the source code and this was a vestige of CL 146999.

Fixes #28781

Change-Id: I3d2ada5f9e8bf4ce4f015b9bd379b311592fe3ce
Reviewed-on: https://go-review.googlesource.com/c/149458
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

test/run.go

@@ -808,7 +808,8 @@ func (t *test) run() {
 
 		}
 		if len(asms) > 0 {
-			if err := ioutil.WriteFile(filepath.Join(longdir, "go_asm.h"), nil, 0666); err != nil {
+			emptyHdrFile := filepath.Join(t.tempDir, "go_asm.h")
+			if err := ioutil.WriteFile(emptyHdrFile, nil, 0666); err != nil {
 				t.err = fmt.Errorf("write empty go_asm.h: %s", err)
 				return
 			}