test/parser: cleanup parser allocations on exit

We should talloc_free the parser list once we're done, enabling
valgrind's leak check. Before:

 [jk@pablo parser]$ valgrind ./test-null
 ...
 ==9330== HEAP SUMMARY:
 ==9330==     in use at exit: 288 bytes in 3 blocks
 ==9330==   total heap usage: 11 allocs, 8 frees, 1,177 bytes allocated

After:

 [jk@pablo parser]$ valgrind ./test-null
 ...
 ==9940== HEAP SUMMARY:
 ==9940==     in use at exit: 0 bytes in 0 blocks
 ==9940==   total heap usage: 11 allocs, 11 frees, 1,177 bytes allocated
 ==9940==
 ==9940== All heap blocks were freed -- no leaks are possible

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>