Having these named differently than the test itself mostly creates
unecessary confusion and makes writing logic against the tests harder
so let's rename the testsuite-xx units and scripts to just use the
test name itself.
Let's make this behave more like all the rest of the meson stuff.
This also is the first step to making it a bit more flexible so we
can define integration tests in different ways as will be seen in
the next commits.
Just use ExecParam directly, as these are all internal to sd-exec now
anyway. Avoids double close when execution fails after FDs are set up
for inheritance and were already re-arranged.
Fixes https://github.com/systemd/systemd/issues/30412
