docs: use lowercase "qemu" and other minor tweaks to test docs

docs: use lowercase "qemu" and other minor tweaks to test docs

We used both "qemu" and "QEMU", let's use the lower-case version everywhere
since it's also the name of the binary and the version that people are
most familiar with.

The stuff under test/ is not only for the integeration tests, but also
for various other test-related stuff, so adjust the docs a bit.