Eagerly throw UndefVarError

This error doesn't need to be shrunk; it's a clear programmer error
not influenced by any of our generated inputs, so there's no sense
in wasting compute by shrinking a "counterexample" to the property.

src/teststate.jl

@@ -40,6 +40,8 @@ function test_function(ts::TestState, tc::TestCase)
     catch e
         # Interrupts are an abort signal, so rethrow
         e isa InterruptException && rethrow()
+        # UndefVarError are a programmer error, so rethrow
+        e isa UndefVarError && rethrow()
         # These are wanted rejections
         e isa TestException && return (false, false)
         # true errors are always interesting