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-network-parser/directives.network

@@ -0,0 +1,187 @@
+[Bridge]
+Cost=
+UseBPDU=
+HairPin=
+UnicastFlood=
+FastLeave=
+Priority=
+AllowPortToBeRoot=
+[Match]
+KernelVersion=
+Type=
+Driver=
+Architecture=
+Path=
+Name=
+Virtualization=
+KernelCommandLine=
+Host=
+MACAddress=
+[Link]
+RequiredForOnline=
+ARP=
+AllMulticast=
+Unmanaged=
+MTUBytes=
+Multicast=
+MACAddress=
+[BridgeFDB]
+VLANId=
+MACAddress=
+[DHCP]
+UseDomains=
+UseRoutes=
+IAID=
+UserClass=
+UseNTP=
+RapidCommit=
+ForceDHCPv6PDOtherInformation=
+UseMTU=
+UseDomainName=
+RouteMetric=
+SendHostname=
+Anonymize=
+VendorClassIdentifier=
+Hostname=
+DUIDType=
+UseHostname=
+UseDNS=
+CriticalConnection=
+DUIDRawData=
+RequestBroadcast=
+ClientIdentifier=
+ListenPort=
+UseTimezone=
+RouteTable=
+[Route]
+Destination=
+Protocol=
+Table=
+Gateway=
+InitialAdvertisedReceiveWindow=
+GatewayOnlink=
+Type=
+InitialCongestionWindow=
+IPv6Preference=
+PreferredSource=
+Scope=
+MTUBytes=
+QuickAck=
+Source=
+Metric=
+[Network]
+IPv6DuplicateAddressDetection=
+IPMasquerade=
+ProxyARP=
+PrimarySlave=
+IPv4LLRoute=
+Address=
+IPv6ProxyNDPAddress=
+IPv6AcceptRA=
+IPv6AcceptRouterAdvertisements=
+DNSSECNegativeTrustAnchors=
+MACVTAP=
+IPv6PrivacyExtensions=
+MACVLAN=
+MulticastDNS=
+LLDP=
+Tunnel=
+Gateway=
+IPv4LL=
+IPVLAN=
+EmitLLDP=
+IPv6MTUBytes=
+IPv4ProxyARP=
+Bridge=
+DNSOverTLS=
+Bond=
+IPv6ProxyNDP=
+DNS=
+ActiveSlave=
+LLMNR=
+DNSSEC=
+IPv6HopLimit=
+IPForward=
+IPv6Token=
+Description=
+VXLAN=
+LinkLocalAddressing=
+ConfigureWithoutCarrier=
+NTP=
+DHCP=
+Domains=
+IPv6PrefixDelegation=
+VLAN=
+DHCPServer=
+BindCarrier=
+VRF=
+[IPv6Prefix]
+Prefix=
+OnLink=
+PreferredLifetimeSec=
+AddressAutoconfiguration=
+ValidLifetimeSec=
+[BridgeVLAN]
+EgressUntagged=
+VLAN=
+PVID=
+[CAN]
+SamplePoint=
+BitRate=
+RestartSec=
+[Address]
+DuplicateAddressDetection=
+AutoJoin=
+PreferredLifetime=
+Address=
+Scope=
+HomeAddress=
+PrefixRoute=
+ManageTemporaryAddress=
+Broadcast=
+Peer=
+Label=
+[RoutingPolicyRule]
+Table=
+IncomingInterface=
+To=
+OutgoingInterface=
+From=
+TypeOfService=
+Priority=
+FirewallMark=
+[IPv6PrefixDelegation]
+RouterPreference=
+DNSLifetimeSec=
+DNS=
+RouterLifetimeSec=
+Domains=
+EmitDNS=
+EmitDomains=
+Managed=
+OtherInformation=
+[IPv6AddressLabel]
+Label=
+Prefix=
+[IPv6AcceptRA]
+UseDomains=
+RouteTable=
+UseDNS=
+[DHCPServer]
+EmitNTP=
+PoolSize=
+PoolOffset=
+Timezone=
+EmitDNS=
+NTP=
+EmitRouter=
+MaxLeaseTimeSec=
+DefaultLeaseTimeSec=
+EmitTimezone=
+DNS=
+[DHCPv4]
+UseHostname=
+UseMTU=
+UseDomainName=
+CriticalConnection=
+UseDNS=