gh-138013: Split SignalsTest from test_io.test_general (#139079)

Increase parallelism by splitting out `SignalsTest` from test_general.
`SignalsTest` takes 24.2 seconds on my dev machine when fully enabled
making it the largest part of `test_io`. Code move done via copy/paste
then tweak imports.

After splitting `test_io.test_general` is down to 10.1 seconds on my dev
box with all parts enabled.