Allow test sharding for e.g. Bazel test sharding feature

This greatly simplifies running Catch2 tests in single binary
in parallel from external test runners. Instead of having to
shard the tests by tags/test names, an external test runner
can now just ask for test shard 2 (out of X), and execute that
in single process, without having to know what tests are actually
in the shard.

Note that sharding also applies to test listing, and happens after
tests were ordered according to the `--order` feature.

tests/ExtraTests/CMakeLists.txt

@@ -8,6 +8,22 @@ project( Catch2ExtraTests LANGUAGES CXX )
 
 message( STATUS "Extra tests included" )
 
+
+add_test(
+  NAME TestShardingIntegration
+  COMMAND ${PYTHON_EXECUTABLE} ${CATCH_DIR}/tests/TestScripts/testSharding.py $<TARGET_FILE:SelfTest>
+)
+
+add_test(
+  NAME TestSharding::OverlyLargeShardIndex
+  COMMAND $<TARGET_FILE:SelfTest>  --shard-index 5 --shard-count 5
+)
+set_tests_properties(
+    TestSharding::OverlyLargeShardIndex
+  PROPERTIES
+    PASS_REGULAR_EXPRESSION "The shard count \\(5\\) must be greater than the shard index \\(5\\)"
+)
+
 # The MinDuration reporting tests do not need separate compilation, but
 # they have non-trivial execution time, so they are categorized as
 # extra tests, so that they are run less.