Only reseed the internal RNG when a test is first entered

This fixes multiple issues with random generators, with the most
important one being that multiple nested generators could return
values from the same sequence, due to internal implementation
details of `GENERATE`, and how they interact with test case
paths.

The cost of doing this is that given this simple `TEST_CASE`,
```cpp
TEST_CASE("foo") {
    auto i = GENERATE(take(10, random(0, 100));
    SECTION("A") {
        auto j = GENERATE(take(10, random(0, 100));
    }
    SECTION("B") {
        auto k = GENERATE(take(10, random(0, 100));
    }
}
```

`k` will have different values between running the test as
a whole, e.g. with `./tests "foo"`, and running only the "B"
section with `./tests "foo" -c "B"`.

I consider this an acceptable cost, because the only alternative
would be very messy to implement, and add a lot of brittle and
complex code for relatively little benefit.

If this calculation changes, we will need to instead walk
the current tracker tree whenever a random generator is being
constructed, check for random generators on the path to root,
and take a seed from them.

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