tal: add test for group growth.

Rusty forgets a git add.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/tal/test/run-groups-grow.c

+#define CCAN_TAL_DEBUG
+#include <ccan/tal/tal.h>
+#include <ccan/tal/tal.c>
+#include <ccan/tap/tap.h>
+
+static size_t num_allocated;
+
+static void *alloc_account(size_t len)
+{
+	num_allocated++;
+	return malloc(len);
+}
+
+static void free_account(void *p)
+{
+	num_allocated--;
+	return free(p);
+}
+
+#define NUM_ALLOCS 1000
+
+int main(void)
+{
+	void *p, *c[NUM_ALLOCS];
+	int i;
+	size_t allocated_after_first;
+
+	plan_tests(1);
+
+	tal_set_backend(alloc_account, NULL, free_account, NULL);
+
+	p = tal(NULL, char);
+	c[0] = tal(p, char);
+
+	allocated_after_first = num_allocated;
+	for (i = 1; i < NUM_ALLOCS; i++)
+		c[i] = tal(p, char);
+
+	/* Now free them all. */
+	for (i = 0; i < NUM_ALLOCS; i++)
+		tal_free(c[i]);
+
+	/* We can expect some residue from having any child, but limited! */
+	ok1(num_allocated <= allocated_after_first);
+	return exit_status();
+}