test(old): emulate test_override('char_avail') using FFI

Add a non-static variable for this, otherwise it'll be really hacky.
This avoid having to rewrite many incsearch tests in Lua.

(cherry picked from commit c925e7b8ba)

src/nvim/getchar.h

@@ -17,6 +17,8 @@ typedef enum {
 
 enum { NSCRIPT = 15, };  ///< Maximum number of streams to read script from
 
+EXTERN bool disable_char_avail_for_testing INIT( = false);
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "getchar.h.generated.h"
 #endif