[test_runner] Create --write-logs dir, add default dir to .gitignore

`python3 tools/test.py --write-results ...` fails to run if the default
directory (logs) doesn't exist. Create the directory if it doesn't
exist.

Update .gitignore with the correct default paths of logs.json and
results.json.

Change-Id: Ib1118387195c3bb30b350ef3748e66b4057c57de
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/269880
Reviewed-by: Alexander Thomas <[email protected]>
Commit-Queue: Ömer Ağacan <[email protected]>

Author: osa1

.gitignore

@@ -102,8 +102,8 @@ tools/xcodebuild
 /crash_logs/
 /build/config/gclient_args.gni
 /pkg/front_end/testcases/old_dills/
-/logs.json
-/results.json
+logs/logs.json
+logs/results.json
 /async_lazy_debug.so
 /dwarf.so
 /dwarf_obfuscate.so

pkg/test_runner/lib/src/test_progress.dart

@@ -635,7 +635,9 @@ class ResultWriter extends EventListener {
   final List<Map> _logs = [];
   final String _outputDirectory;
 
-  ResultWriter(this._outputDirectory);
+  ResultWriter(this._outputDirectory) {
+    Directory(_outputDirectory).createSync(recursive: true);
+  }
 
   void allTestsKnown() {
     // Write an empty result log file, that will be overwritten if any tests