tests: fix a memory leak in lxcpath

tests: fix a memory leak in lxcpath

```
$ sudo ./src/tests/lxc-test-lxcpath

=================================================================
==95911==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 39 byte(s) in 1 object(s) allocated from:
    #0 0x7effafc8d3dd in strdup (/lib/x86_64-linux-gnu/libasan.so.5+0x963dd)
    #1 0x7effaf5a2de6 in lxcapi_config_file_name /home/vagrant/lxc/src/lxc/lxccontainer.c:3190
    #2 0x562961680c30 in main /home/vagrant/lxc/src/tests/lxcpath.c:49
    #3 0x7effae5150b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)

Direct leak of 21 byte(s) in 1 object(s) allocated from:
    #0 0x7effafc8d3dd in strdup (/lib/x86_64-linux-gnu/libasan.so.5+0x963dd)
    #1 0x7effaf5a2de6 in lxcapi_config_file_name /home/vagrant/lxc/src/lxc/lxccontainer.c:3190
    #2 0x56296168115e in main /home/vagrant/lxc/src/tests/lxcpath.c:77
    #3 0x7effae5150b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)

Direct leak of 21 byte(s) in 1 object(s) allocated from:
    #0 0x7effafc8d3dd in strdup (/lib/x86_64-linux-gnu/libasan.so.5+0x963dd)
    #1 0x7effaf5a2de6 in lxcapi_config_file_name /home/vagrant/lxc/src/lxc/lxccontainer.c:3190
    #2 0x562961680f0a in main /home/vagrant/lxc/src/tests/lxcpath.c:63
    #3 0x7effae5150b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)

SUMMARY: AddressSanitizer: 81 byte(s) leaked in 3 allocation(s).
```

Signed-off-by: Evgeny Vereshchagin <evvers@ya.ru>