Run streams unit test in the browser

[rekmarks]

#349 reverted the @ocap/streams unit test environment to jsdom because it's horrible to debug unit tests running in the browser via playwright under SES. However, we would like our unit tests to mimic the production environment to the greatest extent possible. We should run them in the browser by default, but have a configuration for running them under jsdom for development purposes.

[rekmarks]

Some other issues we want to look at:

• Can we use the ses assert shim in the mock endo environment?
  • I tried, and I don't think it's worth it.
• Can we create a "mock endo" mode and a "real endo (lockdown)" mode for our tests?
  • Yes: test: Add development mode #411