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CL 163726 added workarounds to keep the iOS builders happy in
a symlinked temporary dir.

The workarounds also made the tests more realistic and improved
performance. Keep them but also handle symlinks better in the
exec wrapper.

Change-Id: Iaa2c03a1a3fb3aa5aaf62d79d52b63d5d8f11db5
Reviewed-on: https://go-review.googlesource.com/c/164698

Reviewed-by: Bryan C. Mills <bcmills@google.com>