[release-branch.go1.9] misc/cgo/testcshared: use file descriptor 30 for TestUnexportedSymbols

Backport of CL 108537 to 1.9 release branch.

We were using file descriptor 100, which requires the Linux kernel to
grow the fdtable size. That step may sometimes require a long time,
causing the test to fail. Switch to file descriptor 30, which should
not require growing the fdtable.

Updates #23784
Fixes #25278

Change-Id: I19ea6ab1724ec1807643d5111c44631e20be76b0
Reviewed-on: https://go-review.googlesource.com/111996


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>