Add an experimental new way of capturing stdout/stderr

Unlike the relatively non-invasive old way of capturing stdout/stderr,
this new way is also able to capture output from C's stdlib functions
such as `printf`. This is done by redirecting stdout and stderr file
descriptors to a file, and then reading this file back.

This approach has two sizeable drawbacks:
1) Performance, obviously. Previously an installed capture made the
program run faster (as long as it was then discarded), because a call
to `std::cout` did not result in text output to the console. This new
capture method in fact forces disk IO. While it is likely that any
modern OS will keep this file in memory-cache and might never actually
issue the IO to the backing storage, it is still a possibility and
calls to the file system are not free.

2) Nonportability. While POSIX is usually assumed portable, and this
implementation relies only on a very common parts of it, it is no
longer standard C++ (or just plain C) and thus might not be available
on some obscure platforms. Different C libs might also implement the
relevant functions in a less-than-useful ways (e.g. MS's `tmpfile`
generates a temp file inside system folder, so it will not work
without elevated privileges and thus is useless).

These two drawbacks mean that, at least for now, the new capture is
opt-in. To opt-in, `CATCH_CONFIG_EXPERIMENTAL_REDIRECT` needs to be
defined in the implementation file.

Closes #1243

docs/configuration.md

@@ -121,6 +121,7 @@ by using `_NO_` in the macro, e.g. `CATCH_CONFIG_NO_CPP17_UNCAUGHT_EXCEPTIONS`.
     CATCH_CONFIG_DISABLE_STRINGIFICATION    // Disable stringifying the original expression
     CATCH_CONFIG_DISABLE                    // Disables assertions and test case registration
     CATCH_CONFIG_WCHAR                      // Enables use of wchart_t
+    CATCH_CONFIG_EXPERIMENTAL_REDIRECT      // Enables the new (experimental) way of capturing stdout/stderr
 
 Currently Catch enables `CATCH_CONFIG_WINDOWS_SEH` only when compiled with MSVC, because some versions of MinGW do not have the necessary Win32 API support.
 
@@ -131,7 +132,9 @@ Currently Catch enables `CATCH_CONFIG_WINDOWS_SEH` only when compiled with MSVC,
 `CATCH_CONFIG_WCHAR` is on by default, but can be disabled. Currently
 it is only used in support for DJGPP cross-compiler.
 
-These toggles can be disabled by using `_NO_` form of the toggle, e.g. `CATCH_CONFIG_NO_WINDOWS_SEH`.
+With the exception of `CATCH_CONFIG_EXPERIMENTAL_REDIRECT`,
+these toggles can be disabled by using `_NO_` form of the toggle,
+e.g. `CATCH_CONFIG_NO_WINDOWS_SEH`.
 
 ### `CATCH_CONFIG_FAST_COMPILE`
 Defining this flag speeds up compilation of test files by ~20%, by making 2 changes: