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/eigen.py

@@ -11,7 +11,11 @@ from example import sparse_r, sparse_c
 from example import sparse_passthrough_r, sparse_passthrough_c
 from example import double_row, double_col
 from example import double_mat_cm, double_mat_rm
-import numpy as np
+try:
+    import numpy as np
+except ImportError:
+    # NumPy missing: skip test
+    exit(99)
 
 ref = np.array(
     [[0, 3, 0, 0, 0, 11],