Martin Hořeňovský dcafc605f3
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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What's the Catch2?

Catch2 is mainly a unit testing framework for C++, but it also provides basic micro-benchmarking features, and simple BDD macros.

Catch2's main advantage is that using it is both simple and natural. Tests autoregister themselves and do not have to be named with valid identifiers, assertions look like normal C++ code, and sections provide a nice way to share set-up and tear-down code in tests.

Catch2 v3 is being developed!

You are on the devel branch, where the next major version, v3, of Catch2 is being developed. As it is a significant rework, you will find that parts of this documentation are likely still stuck on v2.

For stable (and documentation-matching) version of Catch2, go to the v2.x branch.

For migrating from the v2 releases to v3, you should look at our documentation. It provides a simple guidelines on getting started, and collects most common migration problems.

How to use it

This documentation comprises these three parts:

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Description
A modern, C++-native, test framework for unit-tests, TDD and BDD - using C++14, C++17 and later (C++11 support is in v2.x branch, and C++03 on the Catch1.x branch)
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