test: don't install test-network-generator-conversion.sh w/o networkd

test: don't install test-network-generator-conversion.sh w/o networkd

otherwise TEST-02 will fail:

```
=== Failed test log ===
--- test-network-generator-conversion.sh begin ---
+ [[ -n '' ]]
+ [[ -x /usr/lib/systemd/systemd-network-generator ]]
+ [[ -x /lib/systemd/systemd-network-generator ]]
+ exit 1
--- test-network-generator-conversion.sh end ---
```

Before:
```
$ meson build -Dnetworkd=false -Dinstall-tests=true
$ ninja -C build
$ DESTDIR=$PWD/test-install ninja -C build install
$ find test-install/ -name test-network-generator-conversion.sh
test-install/usr/lib/systemd/tests/test-network-generator-conversion.sh
```

After:
```
$ find test-install/ -name test-network-generator-conversion.sh
<no output>
```