build: misc/cgo/testcshared failure [1.9 backport]

[gopherbot]

@FiloSottile requested issue #23784 to be considered for backport to the next 1.9 minor release.

@ianlancetaylor this breaks the trybots on the 1.10 release branch as well, and is probably (?) also an issue on 1.9, can you look into backporting the fix?

https://go-review.googlesource.com/c/go/+/111715#message-c7b7c5f6de1d1ad25c8c7db15599e99e29a392a7

@gopherbot please open a backport issue for 1.10 and 1.9.

[gopherbot]

Change https://golang.org/cl/111996 mentions this issue: [release-branch.go1.9] misc/cgo/testcshared: use file descriptor 30 for TestUnexportedSymbols

[gopherbot]

Closed by merging 0ce6af9 to release-branch.go1.9.