[libc] Fix vfprintf test colliding with fprintf

[michaelrj]

Summary

Sometimes the vfprintf test was failing, I suspect that's due to it
using the same filename as the fprintf test. This patch fixes that
problem by changing the filename of the vfprintf output file.

libc/test/src/stdio/vfprintf_test.cpp

int call_vfprintf(::FILE *__restrict stream, const char *__restrict format,
   va_list vlist;
   va_start(vlist, format);
   int ret = __llvm_libc::vfprintf(stream, format, vlist);
   va_end(vlist);
   return ret;
 }
 
 TEST(LlvmLibcVFPrintfTest, WriteToFile) {
-  const char *FILENAME = "fprintf_output.test";
+  const char *FILENAME = "vfprintf_output.test";
   auto FILE_PATH = libc_make_test_file_path(FILENAME);
 
   ::FILE *file = printf_test::fopen(FILE_PATH, "w");
   ASSERT_FALSE(file == nullptr);
 
   int written;
 
   constexpr char simple[] = "A simple string with no conversions.\n";