move g_test_dbg_lock_sleep from a global to a function level static

This avoids the need to define that variable in every program
which uses epee.

tests/functional_tests/main.cpp

@@ -55,8 +55,6 @@ namespace
   const command_line::arg_descriptor<size_t> arg_test_repeat_count = {"test_repeat_count", "", 1};
 }
 
-unsigned int epee::g_test_dbg_lock_sleep = 0;
-
 int main(int argc, char* argv[])
 {
   TRY_ENTRY();