Use FAIL_CHECK macro to report non-fatal failures to Catch2

This avoids a two-step dance of first capturing a local variable
and then doing a roundabout CHECK(false);

The previous approach produces a somewhat cluttered error message,
that primarily describes the expression `failure.empty() == false`,
showing the real issue only as an extra "failure := "..." variable
(which is presented as a quoted value, and possible truncated).

FAILED:
  CHECK( failure.empty() )
with expansion:
  false
with message:
  failure := "test.cpp:15
  Unfulfilled expectation:
  Expected mock.foo(42) to be called once, actually never called
    param  _1 == 42
  "

Using FAIL_CHECK puts the real message up front, with no extra quoting.
Like CHECK(false) (but unlike FAIL) it allows execution to continue.

FAILED:
explicitly with message:
  test.cpp:15
  Unfulfilled expectation:
  Expected mock.foo(42) to be called once, actually never called
    param  _1 == 42

FAIL_CHECK has been available since Catch2 1.8.2

Author: puetzk

docs/CookBook.md

@@ -286,8 +286,7 @@ Before running any tests, make sure to call:
     }
     else
     {
-      CAPTURE(failure);
-      CHECK(failure.empty());
+      FAIL_CHECK(failure);
     }
   });
 ```

include/catch2/trompeloeil.hpp

@@ -47,11 +47,9 @@ namespace trompeloeil
     else
     {
 #ifdef CATCH_CONFIG_PREFIX_ALL
-      CATCH_CAPTURE(failure);
-      CATCH_CHECK(failure.empty());
+      CATCH_FAIL_CHECK(failure);
 #else
-      CAPTURE(failure);
-      CHECK(failure.empty());
+      FAIL_CHECK(failure);
 #endif
     }
   }