validation: LinuxUIDMapping: fix tests

Don't validate uid mappings and gid mappings separately: containers with
only user mappings or with only group mappings are not usable.

The tests about uid/gid mappings now pass with runc:

```
$ sudo  validation/linux_uid_mappings.t
TAP version 13
ok 1 - root filesystem
ok 2 - hostname
ok 3 - process
ok 4 - mounts
ok 5 - user
ok 6 - rlimits
ok 7 - capabilities
ok 8 - default symlinks
ok 9 - default file system
ok 10 - default devices
ok 11 - linux devices
ok 12 - linux process
ok 13 - masked paths
ok 14 - oom score adj
ok 1 # SKIP syscall action SCMP_ACT_ALLOW
ok 2 # SKIP syscall action SCMP_ACT_ALLOW
ok 3 # SKIP syscall action SCMP_ACT_ALLOW
ok 4 # SKIP syscall action SCMP_ACT_ALLOW
ok 5 # SKIP syscall action SCMP_ACT_ALLOW
ok 6 # SKIP syscall action SCMP_ACT_ALLOW
ok 15 - seccomp
ok 16 - read only paths
ok 17 - rootfs propagation
ok 18 - sysctls
ok 19 - uid mappings
ok 20 - gid mappings
1..20
```

Signed-off-by: Alban Crequy <alban@kinvolk.io>

Author: alban

validation/linux_gid_mappings.go

@@ -7,7 +7,8 @@ import (
 func main() {
 	g := util.GetDefaultGenerator()
 	g.AddOrReplaceLinuxNamespace("user", "")
-	g.AddLinuxUIDMapping(uint32(1000), uint32(0), uint32(3200))
+	g.AddLinuxUIDMapping(uint32(1000), uint32(0), uint32(2000))
+	g.AddLinuxGIDMapping(uint32(1000), uint32(0), uint32(3000))
 	err := util.RuntimeInsideValidate(g, nil)
 	if err != nil {
 		util.Fatal(err)