Add clock_gettime_heap snippet

This invokes clock_gettime, but uses a timespec from the heap instead
of the stack.

test/snippets/clock_gettime_heap.c

@@ -0,0 +1,8 @@
+struct timespec *ts = malloc(sizeof(struct timespec));
+clockid_t ckid = CLOCK_REALTIME;
+int ret = clock_gettime(ckid, ts);
+if (ret == 0) {
+  printf("[%s] clock_gettime_heap(CLOCK_REALTIME[%d], ts) -> {%lld, %ld}\n", where, ckid, (long long)ts->tv_sec, ts->tv_nsec);
+ } else {
+  printf("[%s] clock_gettime_heap(CLOCK_REALTIME[%d], ts) returned non-zero (%d), errno = %d (%s)\n", where, ckid, ret, errno, strerror(errno));
+ }

test/snippets/clock_gettime_heap.variable

@@ -0,0 +1 @@
+FAKETIME 2020-02-02 02:02:02+00:00

test/snippets/include_headers.h

@@ -6,3 +6,4 @@
 #include <string.h>
 #include <errno.h>
 #include <time.h>
+#include <stdlib.h>