time: fix zoneinfo.zip locating logic when built with -trimpath

time: fix zoneinfo.zip locating logic when built with -trimpath

When the test binary is built with the -trimpath flag,
runtime.GOROOT() is invalid, and must not be used to locate
GOROOT/lib/time/zoneinfo.zip. (We can use other sources instead.)

However, the test for the package expects zoneinfo.zip to definitely
exist. 'go test' runs the test binary in the directory containing its
source code — in this case GOROOT/src/time — so we can use that
information to find the zoneinfo.zip file when runtime.GOROOT isn't
available.

For #51483

Change-Id: I9de35252a988d146b5d746794323214d400e64e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/391814
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>