internal/trace: add Go 1.19 test data

internal/trace: add Go 1.19 test data

Update instructions to match what seems to be the historical practice:
to generate canned traces when a version is finalized, rather than
waiting until it is superseded.

Follow rename of trace-internal tests from "Span" to "Region". Update
the net/http test invocation to match the apparent intent and the actual
http_1_5_good behavior (about 7ms of total run time and trace file size
under 50kB).

Change-Id: Ifd4c85882159478852e0b8f0d771b6f16b8f3c1b
Reviewed-on: https://go-review.googlesource.com/c/go/+/413776
Run-TryBot: Rhys Hiltner <rhys@justin.tv>
Reviewed-by: Carlos Amedee <carlos@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>