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.

src/catch2/internal/catch_commandline.cpp

@@ -149,6 +149,15 @@ namespace Catch {
                 return ParserResult::runtimeError( "Unrecognized reporter, '" + reporter + "'. Check available with --list-reporters" );
             return ParserResult::ok( ParseResultType::Matched );
         };
+        auto const setShardCount = [&]( std::string const& shardCount ) {
+            auto result = Clara::Detail::convertInto( shardCount, config.shardCount );
+
+            if (config.shardCount == 0) {
+                return ParserResult::runtimeError( "The shard count must be greater than 0" );
+            } else {
+                return result;
+            }
+        };
 
         auto cli
             = ExeName( config.processName )
@@ -240,6 +249,12 @@ namespace Catch {
             | Opt( config.benchmarkWarmupTime, "benchmarkWarmupTime" )
                 ["--benchmark-warmup-time"]
                 ( "amount of time in milliseconds spent on warming up each test (default: 100)" )
+            | Opt( setShardCount, "shard count" )
+                ["--shard-count"]
+                ( "split the tests to execute into this many groups" )
+            | Opt( config.shardIndex, "shard index" )
+                ["--shard-index"]
+                ( "index of the group of tests to execute (see --shard-count)" )
             | Arg( config.testsOrTags, "test name|pattern|tags" )
                 ( "which test or tests to use" );