[release-branch.go1.10] misc/cgo/testcshared: use file descriptor 30 for TestUnexportedSymbols

Backport of CL 108537 to 1.10 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 #25277

Change-Id: I9d25986f3b59bdeb04aa52407b24aa94712aedff
Reviewed-on: https://go-review.googlesource.com/111995


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