Implement <spawn.h>.

As described here:

  http://pubs.opengroup.org/onlinepubs/9699919799/functions/posix_spawn.html

And here:

  http://man7.org/linux/man-pages/man3/posix_spawn.3.html

Bug: N/A (but mentioned in my inbox since 2013)
Test: ran tests
Change-Id: I0b27b2919b660779e3bd8a25fb429527c16dc621
diff --git a/tests/utils.h b/tests/utils.h
index daf382e..ba006f1 100644
--- a/tests/utils.h
+++ b/tests/utils.h
@@ -38,6 +38,12 @@
 #define PATH_TO_SYSTEM_LIB "/system/lib/"
 #endif
 
+#if defined(__GLIBC__)
+#define BIN_DIR "/bin/"
+#else
+#define BIN_DIR "/system/bin/"
+#endif
+
 #if defined(__BIONIC__)
 #define KNOWN_FAILURE_ON_BIONIC(x) xfail_ ## x
 #else
@@ -159,6 +165,9 @@
   char** GetArgs() {
     return const_cast<char**>(args_.data());
   }
+  const char* GetArg0() {
+    return args_[0];
+  }
   char** GetEnv() {
     return const_cast<char**>(env_.data());
   }