Compile fix for the testthat 2.1.1 extension in R 3.6.0

[crubb]

(created using eb --new-pr)

Fix for
../inst/include/testthat/vendor/catch.h:6495:33: error: size of array ‘altStackMem’ is not an integral constant-expression

See

• Build of testthat fails r-lib/testthat#1373
• Use a constant value for the stack size r-lib/testthat#1403

[boegel]

This probably also fixes these errors that occur with testthat 3.0.2 in R-4.1.0-foss-2021a.eb:

../inst/include/testthat/vendor/catch.h:6495:41: error: array bound is not an integer constant before ] token
 6495 |         static char altStackMem[SIGSTKSZ];
      |                                         ^
../inst/include/testthat/vendor/catch.h: In constructor Catch::FatalConditionHandler::FatalConditionHandler():
../inst/include/testthat/vendor/catch.h:6514:30: error: altStackMem was not declared in this scope
 6514 |             sigStack.ss_sp = altStackMem;
      |                              ^~~~~~~~~~~
../inst/include/testthat/vendor/catch.h: At global scope:
../inst/include/testthat/vendor/catch.h:6546:53: error: array bound is not an integer constant before ] token
 6546 |

So we should try and apply the same patch to testthat in more recent R easyconfigs as well.

The fix is included in testthat v3.1.0, cfr. r-lib/testthat@c784176