Intercept most exit calls and raise instead

spec/spectator/system_exit_spec.cr

@@ -0,0 +1,13 @@
+require "../spec_helper"
+
+Spectator.describe Spectator::SystemExit do
+  it "is raised when an attempt is made to exit the application" do
+    expect { exit }.to raise_error(described_class)
+  end
+
+  it "has the status code passed to an exit call" do
+    exit 5
+  rescue error : Spectator::SystemExit
+    expect(error.status).to eq(5)
+  end
+end