test: add memmove test

Triggers a bug in MIPS code under certain circumstances, which
are unclear at the moment.

test/string/tst-memmove.c

@@ -0,0 +1,44 @@
+# Copyright (C) 2016 Rene Nielsen <rene.nielsen@microsemi.com>
+# Licensed under the LGPL v2.1, see the file COPYING.LIB in this tarball.
+
+#include <string.h>
+#include <stdio.h>
+
+#define LEN 1024
+
+int main(int argc, char *argv[])
+{
+    unsigned char a[LEN], exp;
+    int i, move_len, move_src, err = 0;
+
+    // Fill the array with increasing values from 0 to LEN - 1 (wrap is fine)
+    for (i = 0; i < LEN; i++) {
+        a[i] = i;
+    }
+
+    // move_src is the index into the array to move from.
+    // move_len is the number of indices to move. Here, we take the remainder.
+    move_src = LEN / 64;
+    move_len = LEN - move_src;
+
+    printf("Moving %d entries from index %d to 0\n", move_len, move_src);
+    memmove(a, &a[move_src], sizeof(a[0]) * move_len);
+
+    // Check that the memmove went well
+    for (i = 0; i < LEN; i++) {
+        // Expect that the first #move_len entries contain increasing values
+        // starting at #move_src and the last #move_src entries are untouched.
+        exp = i >= move_len ? i : i + move_src;
+        if (a[i] != exp) {
+            printf("Error: memmove() failed. Expected a[%d] = %u, got %u\n", i, exp, a[i]);
+            err = 1;
+        }
+    }
+
+    if (!err) {
+       printf("memmove() succeeded\n");
+    }
+
+    return err;
+}
+