Provide default stub for Process.exit

Prevent terminating the process from within a test.

src/spectator/mocks.cr

@@ -1,4 +1,5 @@
 require "./mocks/*"
+require "./system_exit"
 
 module Spectator
   # Functionality for mocking existing types.
@@ -11,3 +12,13 @@ module Spectator
     end
   end
 end
+
+# Add default stub to `exit` method.
+# This captures *most* (technically not all) attempts to exit the process.
+# This stub only takes effect in example code.
+# It intercepts `exit` calls and raises `Spectator::SystemExit` to prevent killing the test.
+class ::Process
+  include ::Spectator::Mocks::Stubs
+
+  stub self.exit(code) { raise ::Spectator::SystemExit.new }
+end

src/spectator/system_exit.cr

@@ -0,0 +1,5 @@
+module Spectator
+  # Exception raised when `exit` is called and intercepted from a stub.
+  class SystemExit < Exception
+  end
+end