Additionally introduce `@playwright/browser-<browser>` packages that
just download the respective browser, but do not export anything.
References #26614.
This patch:
- changes the `childProcess` fixture to reliably SIGKILL all descendants
(children and grand-children, regardless of their process group).
This is achieved using the `ps` command to build the process tree, and
then send
`SIGKILL` to the descendant process groups.
- changes the `runCLI` fixture to **not** auto-close codegen by default;
the `childProcess` fixture will clean up all processes. This makes
sure that all `runCLI.waitFor()` commands actually wait until the
necessary
output.
- for a handful of tests that do actually want to auto-close codegen,
introduce an optional `autoCloseWhen` flag for the `runCLI` fixture
that makes sure to close the codegen once a certain output was reached.
