tests: add a couple of files containing all the sections and directives

tests: add a couple of files containing all the sections and directives

This should help the fuzzers to discover code paths faster.

In case anyone is interested, they were generated with the following script
```
perl -aF'/[\s,]+/' -ne '
if (my ($s, $d) = ($F[0] =~ /^([^\s\.]+)\.([^\s\.]+)$/)) { $d{$s}{$d} = 1; }
END { while (my ($key, $value) = each %d) {
    printf "[%s]\n%s\n", $key, join("\n", keys(%$value))
}}'
```
by passing src/network/networkd-network-gperf.gperf and
src/network/netdev/netdev-gperf.gperf to it.