Fix failing nan_init sequential test

tests/run/parallel.pyx

@@ -42,4 +42,5 @@ def test_num_threads():
 
     return num_threads
 
+OPENMP_PARALLEL = True
 include "sequential_parallel.pyx"

tests/run/sequential_parallel.pyx

@@ -237,6 +237,8 @@ def test_nan_init():
     with nogil, cython.parallel.parallel():
         c1 = 16
 
+    if globals().get('OPENMP_PARALLEL'):
+        # We only reach here when we are included by parallel.pyx
         assert c1 == 20, c1
 
 cdef void nogil_print(char *s) with gil: