test: make unit.h self sufficient

Unit test compilation with `#define UNIT_TAP_COMPATIBLE 1` might fail
with an error complaining that <stdarg.h> is not included. Fix this.

In-scope-of #4264

NO_CHANGELOG=testing stuff
NO_DOC=testing stuff

(cherry picked from commit b9fd4557)

test/unit/unit.h

@@ -136,6 +136,8 @@ int check_plan(void);
 
 #if UNIT_TAP_COMPATIBLE
 
+#include <stdarg.h> /* va_start(), va_end() */
+
 #define header()					\
 	do { 						\
 		_space(stdout);				\