cmd/dist: introduce test variants

This introduces the concept of test variants in dist, which are
different configurations of the same package. The variant of a test is
a short string summarizing the configuration.

The "variant name" of a test is either the package name if the variant
is empty, or package:variant if not. Currently this isn't used for
anything, but soon we'll use this as the Package field of the test
JSON output so that we can disambiguate output from differently
configured runs of the same test package, and naturally flow this
through to any test result viewer.

The long-term plan is to use variant names as dist's own test names
and eliminate the ad hoc names it has right now. Unfortunately, the
build coordinator is aware of many of the ad hoc dist test names, so
some more work is needed to get to that point. This CL keeps almost
all test names the same, with the exception of tests registered by
registerCgoTests, where we regularize test names a bit using variants
to avoid some unnecessary complexity (I believe nothing depends on the
names of these tests).

For #37486.

Change-Id: I119fec2872e40b12c1973cf2cddc7f413d62a48c
Reviewed-on: https://go-review.googlesource.com/c/go/+/495016
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

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