Revert "temporarily disable pre-processor test"

This reverts commit def5bddb62.

test/test_preprocessor.py

@@ -0,0 +1,36 @@
+import pqclean
+import helpers
+
+
+def test_preprocessor():
+    for scheme in pqclean.Scheme.all_schemes():
+        for implementation in scheme.implementations:
+            yield check_preprocessor, implementation
+
+
+@helpers.filtered_test
+def check_preprocessor(implementation: pqclean.Implementation):
+    cfiles = implementation.cfiles()
+    hfiles = implementation.hfiles()
+    errors = []
+    for file in hfiles + cfiles:
+        with open(file) as f:
+            for i, line in enumerate(f):
+                line = line.strip()
+                if file in hfiles and i == 0 and line.startswith('#ifndef'):
+                    continue
+                if line.startswith('#if'):
+                    errors.append("\n at {}:{}".format(file, i+1))
+    if errors:
+        raise AssertionError(
+            "Prohibited use of preprocessor conditional" + "".join(errors)
+        )
+
+
+if __name__ == "__main__":
+    try:
+        import nose2
+        nose2.main()
+    except ImportError:
+        import nose
+        nose.runmodule()