The way `zig test` works is that it uses a stand-in

var test_functions: []const TestFn = undefined;

during semantic analysis, but then just before codegen, it swaps out the
value with a constant like this:

const test_functions: []const TestFn = .{foo, bar, baz, etc};

Before this commit, the `Module.Variable` associated with the stand-in
value was leaked; now it is properly cleaned up before being replaced.