internal/testmath: add two-sample Welch's t-test for performance tests
This CL copies code from github.com/aclements/go-moremath/stats and
github.com/aclements/go-moremath/mathx for Welch's t-test. Several
existing tests in the Go repository check performance and scalability,
and this import is part of a move toward a more rigorous measurement of
both.
Note that the copied code is already licensed to Go Authors, so there's
no need to worry about additional licensing considerations.
For #32986.
Change-Id: I058630fab7216d1a589bb182b69fa2231e6f5475
Reviewed-on: https://go-review.googlesource.com/c/go/+/411395 Reviewed-by: Michael Pratt <mpratt@google.com>