cmd/go: use testGOROOT in TestListTemplateContextFunction

cmd/go: use testGOROOT in TestListTemplateContextFunction

This test uses testgo to run 'go list', so it should use the correct
GOROOT for testgo. (This may be particularly relevant when the test
binary itself is build with -trimpath, in which case runtime.GOROOT()
is not valid.)

Updates #51483

Change-Id: I79b310f88e3a200122d6289073df1385e3e97cca
Reviewed-on: https://go-review.googlesource.com/c/go/+/391801
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Trust: Daniel Martí <mvdan@mvdan.cc>