Build tests/hello_world_file.cpp as part of a test. NFC (#15996)

This file is refernced in the tutorial but nowhere else.  Add a test to
ensure it continues to be buildable.

Author: sbc100

tests/hello_world_file.cpp

@@ -4,6 +4,7 @@
 // found in the LICENSE file.
 
 #include <stdio.h>
+
 int main() {
   FILE *file = fopen("tests/hello_world_file.txt", "rb");
   if (!file) {

tests/test_other.py

@@ -11449,3 +11449,7 @@ def test_llvm_nm_relative_paths_works_with_response_files(self):
       }
     ''')
     self.run_process([EMCC, 'main.c', os.path.join('foo', 'foo bar.c')])
+
+  def test_tutorial(self):
+    # Ensure that files referenced in Tutorial.rst are buildable
+    self.run_process([EMCC, test_file('hello_world_file.cpp')])