unittest behaviour

[DLearner]

Please consider:

```
size_t foo() {
    static size_t var1 = 1;

    var1 = var1 + 1;
    return var1;
}

unittest {

    assert(foo() == 2);
    assert(foo() == 3);
}

```

which works as expected.

But

```
size_t foo1() {
    static size_t var1 = 1;

    var1 = var1 + 1;
    return var1;
}

unittest {
    assert(foo1() == 2);
}

unittest {
    assert(foo1() == 2);
}

```

Fails on the second unittest.

I appreciate this behaviour matches the docs (so not a bug), but 
is it desirable?

To me, as a test harness, a umittest block should be a completely 
fresh-from-scratch invocation of the code inside the block, and 
thus not depend on the result/effects of any previous unittest.