test: add error return to memcpy_len_mismatch and memcpy_alias

test/cases/safety/memcpy_alias.zig

@@ -12,6 +12,7 @@ pub fn main() !void {
     var len: usize = 5;
     _ = &len;
     @memcpy(buffer[0..len], buffer[4 .. 4 + len]);
+    return error.TestFailed;
 }
 // run
 // backend=stage2,llvm

test/cases/safety/memcpy_len_mismatch.zig

@@ -12,6 +12,7 @@ pub fn main() !void {
     var len: usize = 5;
     _ = &len;
     @memcpy(buffer[0..len], buffer[len .. len + 4]);
+    return error.TestFailed;
 }
 // run
 // backend=stage2,llvm