Add a unit test for tor_spawn_background

- Test sucessfully starting a process
- Test failing to find the executable

src/common/util.c

@@ -2973,7 +2973,7 @@ format_helper_exit_status(unsigned char child_state, int saved_errno,
  * -1.  Some parts of this code are based on the POSIX subprocess module from
  * Python.
  */
-static int
+int
 tor_spawn_background(const char *const filename, int *stdout_read,
                      int *stderr_read, const char **argv)
 {

src/common/util.h

@@ -350,6 +350,8 @@ HANDLE load_windows_system_library(const TCHAR *library_name);
 
 #ifdef UTIL_PRIVATE
 /* Prototypes for private functions only used by util.c (and unit tests) */
+int tor_spawn_background(const char *const filename, int *stdout_read,
+                         int *stderr_read, const char **argv);
 void format_helper_exit_status(unsigned char child_state,
                                int saved_errno, char *hex_errno);