Allow CATCH_CONFIG_DEFAULT_REPORTER to be arbitrary reporter spec

Previously it could be just plain reporter name, e.g. `xml`, but
it could not specify other reporter options. This change is not
particularly useful for the built-in reporters, as it mostly comes
in handy for combining specific custom reporter with custom arguments,
and the built-in reporters do not have those.

src/catch2/catch_config.cpp

@@ -107,14 +107,16 @@ namespace Catch {
 
         // Insert the default reporter if user hasn't asked for a specific one
         if ( m_data.reporterSpecifications.empty() ) {
-            m_data.reporterSpecifications.push_back( {
 #if defined( CATCH_CONFIG_DEFAULT_REPORTER )
-                CATCH_CONFIG_DEFAULT_REPORTER,
+            const auto default_spec = CATCH_CONFIG_DEFAULT_REPORTER;
 #else
-                "console",
+            const auto default_spec = "console";
 #endif
-                {}, {}, {}
-            } );
+            auto parsed = parseReporterSpec(default_spec);
+            CATCH_ENFORCE( parsed,
+                           "Cannot parse the provided default reporter spec: '"
+                               << default_spec << '\'' );
+            m_data.reporterSpecifications.push_back( std::move( *parsed ) );
         }
 
         if ( enableBazelEnvSupport() ) {

tests/TestScripts/testConfigureDefaultReporter.py

@@ -28,14 +28,20 @@
 catch2_source_path = os.path.abspath(sys.argv[1])
 build_dir_path = os.path.join(os.path.abspath(sys.argv[2]), 'CMakeConfigTests', 'DefaultReporter')
 
+# TODO: incorporate build_dir_path in the output filename
+output_file = f"{build_dir_path}/foo.xml"
 configure_and_build(catch2_source_path,
                     build_dir_path,
-                    [("CATCH_CONFIG_DEFAULT_REPORTER", "xml")])
+                    [("CATCH_CONFIG_DEFAULT_REPORTER", f"xml::out={output_file}")])
 
 stdout, _ = run_and_return_output(os.path.join(build_dir_path, 'tests'), 'SelfTest', ['[approx][custom]'])
 
-xml_tag = '</Catch2TestRun>'
-if xml_tag not in stdout:
-    print("Could not find '{}' in the stdout".format(xml_tag))
-    print('stdout: "{}"'.format(stdout))
+if not os.path.exists(output_file):
+    print(f'Did not find the {output_file} file')
     exit(2)
+
+xml_tag = '</Catch2TestRun>'
+with open(output_file, 'r', encoding='utf-8') as file:
+    if xml_tag not in file.read():
+        print(f"Could not find '{xml_tag}' in the file")
+        exit(3)