title: Do not combine generators in Catch2 test case
body: <p>I would like to use to more than one generator in a <code>Catch2</code> test case. My problem is, that when there are more than two generators, they are "combined". So if I write something like this:</p>
  
  <pre><code>TEST_CASE("Test legal moves on empty 2x1 board") {
      Board board(2, 1);
  
      auto startPos = GENERATE(Position(0, 0), Position(1, 0));
      auto expectedMoves = GENERATE(Position(1, 0), Position(0, 0));
      auto legalMoves = board.getLegalMoves(startPos);
      REQUIRE(legalMoves[0] == expectedMoves);
  }
  </code></pre>
  
  <p>This will result in four test cases:
  (<code>startPos[0] vs expectedMoves[0]</code>, <code>startPos[0] vs expectedMoves[1]</code>, <code>startPos[1] vs expectedMoves[0]</code>, <code>startPos[1] vs startPos[1]</code>).</p>
  
  <p>I however want only two: (<code>startPos[0] vs expectedMoves[0]</code>, <code>startPos[1] vs startPos[1]</code>).</p>
  
  <p><strong>Is there a way to do it in an elegant and readable way?</strong> I would like to avoid things like this:</p>
  
  <pre><code>TEST_CASE("Test legal moves on empty 2x1 board") {
      Board board(2, 1);
  
      auto dump = GENERATE(vector&lt;Position&gt;{ Position(0, 0), Position(1, 0) },    
                           vector&lt;Position&gt;{ Position(1, 0), Position(0, 0) }); 
      auto expectedMoves = dump[1];       
      auto legalMoves = board.getLegalMoves(dump[0]);       
      REQUIRE(legalMoves[0] == expectedMoves);
  }
  </code></pre>
  
  <p>As maintenance of such monstrosity is cumbersome.</p>
  
  <p>EDIT: I do not really like this solution:
  <a href="https://github.com/catchorg/Catch2/blob/ce42deb72fab2be85a862f559984580c24cb76c4/projects/SelfTest/UsageTests/Generators.tests.cpp#L199" rel="nofollow noreferrer">https://github.com/catchorg/Catch2/blob/ce42deb72fab2be85a862f559984580c24cb76c4/projects/SelfTest/UsageTests/Generators.tests.cpp#L199</a></p>