Fix memory leak

[VTroyanGolovyan]

Fix memory leak:

leaks Report Version: 4.0, multi-line stacks
Process 8686: 211 nodes malloced for 40 KB
Process 8686: 1 leak for 32 total leaked bytes.

STACK OF 1 INSTANCE OF 'ROOT LEAK: <malloc in ctest__get_timestamp>':
4   dyld                                  0x1886f5d54 start + 7184
3   a.out                                 0x102e044b4 main + 20  test.c:31
2   a.out                                 0x102e04710 ctest__run_tests + 584  ctest.h:110
1   a.out                                 0x102e04960 ctest__get_timestamp + 20  ctest.h:121
0   libsystem_malloc.dylib                0x1888ebf44 _malloc_zone_malloc_instrumented_or_legacy + 152 
====
    1 (32 bytes) ROOT LEAK: <malloc in ctest__get_timestamp 0xa7f028020> [32]