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.

test/fuzz/fuzz-netdev-parser/directives.netdev

@@ -0,0 +1,153 @@
+[VLAN]
+MVRP=
+LooseBinding=
+ReorderHeader=
+Id=
+GVRP=
+[MACVLAN]
+Mode=
+[WireGuard]
+ListenPort=
+PrivateKey=
+FwMark=
+[MACVTAP]
+Mode=
+[Match]
+Architecture=
+Host=
+KernelVersion=
+Virtualization=
+KernelCommandLine=
+[GENEVE]
+DestinationPort=
+TTL=
+UDP6ZeroCheckSumTx=
+TOS=
+Id=
+UDPChecksum=
+UDP6ZeroChecksumTx=
+FlowLabel=
+UDP6ZeroChecksumRx=
+Remote=
+UDP6ZeroCheckSumRx=
+[Bridge]
+ForwardDelaySec=
+HelloTimeSec=
+MaxAgeSec=
+STP=
+MulticastSnooping=
+DefaultPVID=
+MulticastQuerier=
+AgeingTimeSec=
+Priority=
+GroupForwardMask=
+VLANFiltering=
+[VRF]
+TableId=
+Table=
+[Peer]
+MACAddress=
+Name=
+[WireGuardPeer]
+Endpoint=
+PresharedKey=
+PersistentKeepalive=
+PublicKey=
+AllowedIPs=
+[Tunnel]
+FooOverUDP=
+IPv6FlowLabel=
+Remote=
+AllowLocalRemote=
+Local=
+TOS=
+Independent=
+Key=
+InputKey=
+Encapsulation=
+Mode=
+OutputKey=
+DiscoverPathMTU=
+FOUDestinationPort=
+CopyDSCP=
+EncapsulationLimit=
+TTL=
+FOUSourcePort=
+[VXLAN]
+UDP6ZeroChecksumRx=
+ARPProxy=
+FlowLabel=
+Local=
+RemoteChecksumTx=
+RouteShortCircuit=
+Remote=
+L3MissNotification=
+Group=
+FDBAgeingSec=
+MacLearning=
+Id=
+RemoteChecksumRx=
+TOS=
+L2MissNotification=
+UDP6ZeroChecksumTx=
+UDP6ZeroCheckSumRx=
+UDPCheckSum=
+GroupPolicyExtension=
+MaximumFDBEntries=
+TTL=
+DestinationPort=
+ReduceARPProxy=
+PortRange=
+UDPChecksum=
+UDP6ZeroCheckSumTx=
+[VXCAN]
+Peer=
+[Bond]
+LearnPacketIntervalSec=
+TransmitHashPolicy=
+MIIMonitorSec=
+PacketsPerSlave=
+DownDelaySec=
+PrimaryReselectPolicy=
+ResendIGMP=
+ARPAllTargets=
+AdActorSystemPriority=
+UpDelaySec=
+AdSelect=
+AdActorSystem=
+AdUserPortKey=
+FailOverMACPolicy=
+ARPValidate=
+Mode=
+GratuitousARP=
+ARPIPTargets=
+MinLinks=
+LACPTransmitRate=
+ARPIntervalSec=
+AllSlavesActive=
+[FooOverUDP]
+Protocol=
+Port=
+Encapsulation=
+[Tap]
+MultiQueue=
+OneQueue=
+User=
+Group=
+PacketInfo=
+VNetHeader=
+[IPVLAN]
+Mode=
+Flags=
+[Tun]
+OneQueue=
+MultiQueue=
+PacketInfo=
+Group=
+User=
+[NetDev]
+Kind=
+MACAddress=
+MTUBytes=
+Description=
+Name=