Add fuzz testing for waypoint solvers.

This fuzz test generates random inputs for waypoint solvers to check if
they can find a solution. If they can't, the debug info for the solver
is dumped to the testcases directory. Another test loads those test
cases, creates a solver from them, and checks that a solution is found.
Obviously it won't be immediately, but it's a starting point for fixing
the bug and serves as a regression test afterward.

pytest.ini

@@ -0,0 +1,9 @@
+[pytest]
+markers =
+    fuzztest: marks tests as fuzz tests
+
+# Disable fuzz tests by default. They're randomized so flaky by nature. They
+# are typically run manually after making changes to fuzzed code to generate
+# new regression tests.
+addopts =
+    -m "not fuzztest"