Add some nop and jmp tests, and fix a hazard with jmpi/calli

tests/jmpi.c

@@ -0,0 +1,21 @@
+#include "test.h"
+
+static int tail(void) { return 42; }
+
+static void
+run_test(jit_state_t *j, uint8_t *arena_base, size_t arena_size)
+{
+  jit_begin(j, arena_base, arena_size);
+
+  jit_jmpi(j, tail);
+
+  int (*f)(void) = jit_end(j, NULL);
+
+  ASSERT(f() == 42);
+}
+
+int
+main (int argc, char *argv[])
+{
+  return main_helper(argc, argv, run_test);
+}