[libc][NFC] Use ASSERT_FP_EQ to comapre NaN values in tests.

This is a continuation of the previous CL which did a similar change in
other tests. To elaborate a little about why we need this - under C++
compilation with headers not from LLVM libc, libraries like libc++ and
libstdc++ provide their own math.h which undefine macros like `isnan`
and provide the overloaded C++ isnan functions which return a boolean
value instead of an integer value returned by the isnan macro.