[libc][minor] Fix assertion in LlvmLibcFILETest.SimpleFileOperations (#126109)

The file descriptor of the first opened file is not necessarily 3, so we
change the assertion so that it's >= 0 (i.e. not an error.)

Fixes #126106

Author: alanzhao1

libc/test/src/stdio/fileop_test.cpp

@@ -31,7 +31,7 @@ TEST(LlvmLibcFILETest, SimpleFileOperations) {
   constexpr char FILENAME[] = "testdata/simple_operations.test";
   ::FILE *file = LIBC_NAMESPACE::fopen(FILENAME, "w");
   ASSERT_FALSE(file == nullptr);
-  ASSERT_EQ(LIBC_NAMESPACE::fileno(file), 3);
+  ASSERT_GE(LIBC_NAMESPACE::fileno(file), 0);
   constexpr char CONTENT[] = "1234567890987654321";
   ASSERT_EQ(sizeof(CONTENT) - 1,
             LIBC_NAMESPACE::fwrite(CONTENT, 1, sizeof(CONTENT) - 1, file));