[libc] Check signs instead of values in memcmp unittests.

[cheng.w]

Summary

The C standard only guarantees the sign of return value. The exact return
value is implementation defined.

[gchatelet]

Using z instead of b or c does not bring much but I don't mind the change.
Thx for the patch.

libc/test/src/string/memcmp_test.cpp

 //===-- Unittests for memcmp ----------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "src/string/memcmp.h"
 #include "utils/UnitTest/Test.h"
 
 TEST(LlvmLibcMemcmpTest, CmpZeroByte) {
   const char *lhs = "ab";
-  const char *rhs = "bc";
+  const char *rhs = "yz";
   EXPECT_EQ(__llvm_libc::memcmp(lhs, rhs, 0), 0);
 }
 
 TEST(LlvmLibcMemcmpTest, LhsRhsAreTheSame) {
   const char *lhs = "ab";
   const char *rhs = "ab";
   EXPECT_EQ(__llvm_libc::memcmp(lhs, rhs, 2), 0);
 }
 
 TEST(LlvmLibcMemcmpTest, LhsBeforeRhsLexically) {
   const char *lhs = "ab";
-  const char *rhs = "ac";
-  EXPECT_EQ(__llvm_libc::memcmp(lhs, rhs, 2), -1);
+  const char *rhs = "az";
+  EXPECT_LT(__llvm_libc::memcmp(lhs, rhs, 2), 0);
 }
 
 TEST(LlvmLibcMemcmpTest, LhsAfterRhsLexically) {
-  const char *lhs = "ac";
+  const char *lhs = "az";
   const char *rhs = "ab";
-  EXPECT_EQ(__llvm_libc::memcmp(lhs, rhs, 2), 1);
+  EXPECT_GT(__llvm_libc::memcmp(lhs, rhs, 2), 0);
 }
 
 TEST(LlvmLibcMemcmpTest, Sweep) {
   static constexpr size_t kMaxSize = 1024;
   char lhs[kMaxSize];
   char rhs[kMaxSize];
 
   const auto reset = [](char *const ptr) {
     for (size_t i = 0; i < kMaxSize; ++i)
       ptr[i] = 'a';
   };
 
   reset(lhs);
   reset(rhs);
   for (size_t i = 0; i < kMaxSize; ++i)
     EXPECT_EQ(__llvm_libc::memcmp(lhs, rhs, i), 0);
 
   reset(lhs);
   reset(rhs);
   for (size_t i = 0; i < kMaxSize; ++i) {
-    rhs[i] = 'b';
-    EXPECT_EQ(__llvm_libc::memcmp(lhs, rhs, kMaxSize), -1);
+    rhs[i] = 'z';
+    EXPECT_LT(__llvm_libc::memcmp(lhs, rhs, kMaxSize), 0);
     rhs[i] = 'a';
   }
 }