[libc] Fix vfprintf test colliding with fprintf

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.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D157523

libc/test/src/stdio/vfprintf_test.cpp

@@ -47,7 +47,7 @@ int call_vfprintf(::FILE *__restrict stream, const char *__restrict format,
 }
 
 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");