test: use a unique machine name for each nspawn test

to avoid scope clashing in case some previous test crashed and/or didn't
clean up properly. Currently all test machines are called `root`, since
the name is automagically derived from the container path (in this case
`/var/tmp/systemd-test.XXXXXX/root`).

E.g. (from Ubuntu CI):
```
[23:10:12] --x-- Running TEST-71-HOSTNAME --x--
make: Entering directory '/tmp/autopkgtest.5LjnBV/build.0mE/systemd/test/TEST-71-HOSTNAME'
+ make -C TEST-71-HOSTNAME setup run
TEST-71-HOSTNAME SETUP: test hostnamed
Reusing existing cached image /tmp/autopkgtest.5LjnBV/build.0mE/systemd/test/TEST-71-HOSTNAME/../default.img → /tmp/autopkgtest.5LjnBV/build.0mE/systemd/test/default.img
'/var/tmp/systemd-test.1yy2SS/default.img' -> '/tmp/autopkgtest.5LjnBV/build.0mE/systemd/test/default.img'
I: Masking supporting services
'/var/tmp/systemd-test.1yy2SS/root/etc/systemd/system/systemd-hwdb-update.service' -> '/dev/null'
'/var/tmp/systemd-test.1yy2SS/root/etc/systemd/system/systemd-journal-catalog-update.service' -> '/dev/null'
'/var/tmp/systemd-test.1yy2SS/root/etc/systemd/system/systemd-networkd.service' -> '/dev/null'
'/var/tmp/systemd-test.1yy2SS/root/etc/systemd/system/systemd-networkd.socket' -> '/dev/null'
'/var/tmp/systemd-test.1yy2SS/root/etc/systemd/system/systemd-resolved.service' -> '/dev/null'
TEST-71-HOSTNAME RUN: test hostnamed
+ env --unset=UNIFIED_CGROUP_HIERARCHY --unset=SYSTEMD_NSPAWN_UNIFIED_HIERARCHY timeout --foreground 1200 /bin/systemd-nspawn --register=no --kill-signal=SIGKILL --directory=/var/tmp/systemd-test.1yy2SS/root --setenv=SYSTEMD_UNIT_PATH=/usr/lib/systemd/tests/testdata/testsuite-71.units:/usr/lib/systemd/tests/testdata/units: /lib/systemd/systemd systemd.unit=testsuite.target systemd.wants=testsuite-71.service systemd.wants=end.service
Spawning container root on /var/tmp/systemd-test.1yy2SS/root.
Press ^] three times within 1s to kill container.
Failed to allocate scope: Unit root.scope already exists.
E: nspawn failed with exit code 1
```

test/test-functions

@@ -538,6 +538,7 @@ run_nspawn() {
         "--kill-signal=SIGKILL"
         "--directory=${1:?}"
         "--setenv=SYSTEMD_UNIT_PATH=/usr/lib/systemd/tests/testdata/testsuite-$2.units:/usr/lib/systemd/tests/testdata/units:"
+        "--machine=TEST-$TESTID"
     )
     local kernel_params=(
         "$PATH_TO_INIT"