Add an xtask test

[kpreid]

Is your feature request related to a problem? Please describe.
It's hard to remember how to run thorough local tests (or even builds) when working on changes that are platform-dependent. CI can do it but CI is slower and requires more steps.

Describe the solution you'd like
Add a test subcommand to the xtask that will run all, or a requested subset of, native and wasm tests, with features such as webgl both enabled and disabled.

[cwfitzgerald]

#3873 includes an xtask for running tests.