tests: make enableBig an option

This makes it possible to identify the test as big in the test file
itself. It also seems like a more natural way to control the test
running.

tests/big-test.nix

@@ -0,0 +1,13 @@
+{ lib, ... }:
+
+{
+  options.test.enableBig = lib.mkOption {
+    type = lib.types.bool;
+    default = true;
+    description = ''
+      Whether to enable <quote>big</quote> tests. These are tests that require
+      more resources than typical tests. For example, tests that depend on large
+      packages or tests that take long to run.
+    '';
+  };
+}