Don't pass --rng-seed in catch_discover_tests

Somehow it took me *that* long to remember that catch_discover_tests is
used to run one test per execution of the test executable with ctest,
which makes seeding the PRNG for the test run order useless.

Author: Morwenn

tests/CMakeLists.txt

@@ -305,8 +305,7 @@ endif()
 
 include(CTest)
 
-string(RANDOM LENGTH 6 ALPHABET 123456789 RNG_SEED)
-catch_discover_tests(main-tests EXTRA_ARGS --rng-seed ${RNG_SEED})
+catch_discover_tests(main-tests)
 if (NOT "${CPPSORT_SANITIZE}" MATCHES "address|memory")
-    catch_discover_tests(heap-memory-exhaustion-tests EXTRA_ARGS --rng-seed ${RNG_SEED})
+    catch_discover_tests(heap-memory-exhaustion-tests)
 endif()