tests/slicing/sources/bitcast{2,3}: do not rely on int having size 4

Use 0x1b instead of 0xab as left shift of 0xab by 24 places cannot be
represented in signed int.

tests/slicing/sources/bitcast2.c

@@ -1,15 +1,15 @@
-_Static_assert(sizeof(int) == 4, "This test assumes sizeof(int) == 4");
+#include <limits.h>
 
 /* test accessing bytes in int */
 int main(void) {
-    int a;
-    a = 0;
+    int a = 0;
     char *byte = (char *) &a;
-    byte[0] = 0xab;
-    byte[1] = 0xab;
-    byte[2] = 0xab;
-    byte[3] = 0xab;
-    test_assert(a == 0xabababab);
+
+    int result = 0;
+    for (unsigned i = 0; i < sizeof(int); i++)
+        result |= (byte[i] = 0x1b) << i * CHAR_BIT;
+
+    test_assert(a == result);
 
     return 0;
 }

tests/slicing/sources/bitcast3.c

@@ -1,19 +1,19 @@
-_Static_assert(sizeof(int) == 4, "This test assumes sizeof(int) == 4");
+#include <limits.h>
 
 union BYTE {
     int i;
-    char b[4];
+    char b[sizeof(int)];
 };
 
 /* test accessing bytes in int */
 int main(void) {
-    union BYTE B;
-    B.i = 0;
-    B.b[0] = 0xab;
-    B.b[1] = 0xab;
-    B.b[2] = 0xab;
-    B.b[3] = 0xab;
-    test_assert(B.i == 0xabababab);
+    union BYTE B = {.i = 0};
+
+    int result = 0;
+    for (unsigned i = 0; i < sizeof(int); i++)
+        result |= (B.b[i] = 0x1b) << i * CHAR_BIT;
+
+    test_assert(B.i == result);
 
     return 0;
 }