intmap: add exhaustive testcases for intmap_after

We can't do the full range, but we can for a handful of bits (8).
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/intmap/test/run-after-exhaustive.c

+#include <ccan/bitops/bitops.h>
+#include <ccan/intmap/intmap.c>
+#include <ccan/tap/tap.h>
+#include <stdio.h>
+
+#define ELEMENTS 8
+
+int main(void)
+{
+	UINTMAP(void *) umap;
+
+	plan_tests((1 << ELEMENTS) * ELEMENTS);
+
+	/* Run through every combination of elements */
+	for (int i = 0; i < (1 << ELEMENTS); i++) {
+		/* Set up map */
+		uintmap_init(&umap);
+		for (int j = 0; j < ELEMENTS; j++) {
+			if ((1 << j) & i)
+				uintmap_add(&umap, j, &umap);
+		}
+
+		/* Try each uintmap_after value */
+		for (int j = 0; j < ELEMENTS; j++) {
+			intmap_index_t idx = j, next;
+
+			if ((i >> (j + 1)) == 0)
+				next = 0;
+			else
+				next = j + 1 + bitops_ls32(i >> (j + 1));
+
+			if (!uintmap_after(&umap, &idx))
+				idx = 0;
+			ok1(idx == next);
+		}
+		uintmap_clear(&umap);
+	}
+
+	/* This exits depending on whether all tests passed */
+	return exit_status();
+}