[libc][NFC] Use ASSERT_FP_EQ to compare nan values in tests.

This change "fixes" one of the uses that was missed in
0524da67b4.

libc/test/src/math/RemQuoTest.h

@@ -123,7 +123,7 @@ public:
       // In normal range on x86 platforms, the long double implicit 1 bit can be
       // zero making the numbers NaN. Hence we test for them separately.
       if (isnan(x) || isnan(y)) {
-        ASSERT_NE(isnan(result.f), 0);
+        ASSERT_FP_EQ(result.f, nan);
         continue;
       }