testing: panic on calls to Short/Verbose before Parse
testing: panic on calls to Short/Verbose before Parse

CL 121936 added this diagnostic to avoid a panic accessing *short.
(Hence the "This shouldn't really be a panic" comment.)

That CL was right to produce a clearer error than a plain memory fault,
but I think wrong to print+exit instead of panicking. I just ran into
one of these in a real program, and there is no indication anywhere
of how the program reached this point. The panic will show that.
So change print+exit to a panic with a helpful message, in contrast
to the original panic with an unhelpful message and the current
helpful message without stack trace.

Change-Id: Ib2bae1dead4ccde92f00fa3a34c05241ff7690c6
Reviewed-on: https://go-review.googlesource.com/c/go/+/177419
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

akutz/go,nawawi/go,CAFxX/go,golang/go,cloudflare/go,akutz/go,cloudflare/go,nawawi/go,nawawi/go,cloudflare/go,golang/go,akutz/go,golang/go,nawawi/go,cloudflare/go,akutz/go,golang/go,cloudflare/go,golang/go,cloudflare/go,CAFxX/go,CAFxX/go,akutz/go,akutz/go,cloudflare/go,CAFxX/go,CAFxX/go,akutz/go,golang/go,golang/go,akutz/go,CAFxX/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,nawawi/go,nawawi/go,cloudflare/go,nawawi/go