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/SelfTest/IntrospectiveTests/CmdLine.tests.cpp

@@ -569,6 +569,27 @@ TEST_CASE( "Process can be configured on command line", "[config][command-line]"
             REQUIRE(config.benchmarkWarmupTime == 10);
         }
     }
+
+    SECTION("Sharding options") {
+        SECTION("shard-count") {
+            CHECK(cli.parse({ "test", "--shard-count=8"}));
+
+            REQUIRE(config.shardCount == 8);
+        }
+
+        SECTION("shard-index") {
+            CHECK(cli.parse({ "test", "--shard-index=2"}));
+
+            REQUIRE(config.shardIndex == 2);
+        }
+
+        SECTION("Zero shard-count") {
+            auto result = cli.parse({ "test", "--shard-count=0"});
+
+            CHECK( !result );
+            CHECK_THAT( result.errorMessage(), ContainsSubstring( "The shard count must be greater than 0" ) );
+        }
+    }
 }
 
 TEST_CASE("Test with special, characters \"in name", "[cli][regression]") {