Tests can skip by exiting with 99; fix eigen test failure

This allows (and changes the current examples) to exit with status 99 to
skip a test instead of outputting a special string ("NumPy missing").

This also fixes the eigen test, which currently fails when eigen
headers are available but NumPy is not, to skip instead of failing when
NumPy isn't available.

example/example7.py

@@ -8,8 +8,8 @@ from example import Matrix
 try:
     import numpy as np
 except ImportError:
-    print('NumPy missing')
-    exit(0)
+    # NumPy missing: skip test
+    exit(99)
 
 m = Matrix(5, 5)