Seperate `REQUIRE` (total show-stopper) for checking unit-test integrity

Sometimes unit tests need to perform their own calculations to compare against those of the tested code. For example, in my use-case, I check that an optimised simulator is producing the same results as a slow direct mathematical evaluation.

This often requires some quick checks of the internal testing code, to check pre- and post-conditions of the helper functions, to help distinguish a bad test from an error in the tested code. This is especially helpful for external contributors writing their own unit tests which use the helper funcitons.

Right now, I use `REQUIRE` in the testing, and in checking the integrity of the internal code. Here's an example; a unit-test which performs its own matrix calculations in order to replicate a tested code's result (which is computed in an entirely different way), and utilises internal testing code:
```C++
vector> getReferenceSum(vector> a, vector> b) {
REQUIRE( a.size() == b.size() );
vector> s = a;
for (size_t r=0; r> a = ...
vector> b = ....

vector> librarySum = superAwesomeFancyFunc(a, b);
vector> referenceSum = getReferenceSum(a, b);

REQUIRE( librarySum == referenceSum );
}
```

This means though that in the ultimately performed unit-test, the checking of pre & post-conditions of the helper functions is included in the statistics of the unit-tests (total number of assertions passed).

Instead, it would be great if there was a seperate "meta" macro, which asserts conditions the test-author believes is tautological, and which when failed (indicating bad unit-testing), stops all testing.

E.g. something like

```C++
DEMAND( cond )
```

Is there currently any way to do this, or at least to seperate the statistics of 'internal' `REQUIRE`s from the 'testing' `REQUIRE`s?
Otherwise, is there a way to write custom macros and 'hook' them into Catch's control-flow (rather than just having them force-exit)?

The issue names no repository files or tests. Start by reviewing Catch2's existing REQUIRE implementation and assertion statistics/control-flow hooks; done would require a documented design and implementation for a separate meta assertion mechanism or an equivalent supported way to exclude internal checks from test statistics.

Written by the indexing model from the issue text.