crypto/x509: make TestIssue51759 builder only

crypto/x509: make TestIssue51759 builder only

Since we can't gate tests on the macOS version on normal machines,
restrict TestIssue51759 to only run on builders, where we have a way to
do this.

Change-Id: I70fc83c587689b499b6a38864973a77bb3e52596
Reviewed-on: https://go-review.googlesource.com/c/go/+/472619
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Roland Shoemaker <roland@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>