Stop integration tests from running every ninja invocation.

It was previously running (in integration_tests/simple) rm -rf out && gn gen out.
This changed the mtime of the integration_tests/simple directory, triggering a rebuild.
By instead doing rm -rf out/*, we prevent the mtime of the integration
test source code from changing.

Change-Id: I3cdc6c2822819ea7bb0835b025c38da86a6a6964
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/27980
Reviewed-by: Takuto Ikuta <tikuta@google.com>
Commit-Queue: Matt Stark <msta@google.com>

tools/clean.py

diff --git a/tools/clean.py b/tools/clean.py
index 48be64d..88c1ca2 100644
--- a/tools/clean.py
+++ b/tools/clean.py
@@ -3,11 +3,20 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-"""A cross-platform rm -rf"""
+"""A cross-platform rm -rf path/to/dir/*.
+
+We specifically don't do rm -rf path/to/dir, because that would change the
+mtime of path/to, triggering ninja to rebuild.
+"""
 
 import os
 import shutil
 import sys
 
-if os.path.exists(sys.argv[1]):
-  shutil.rmtree(sys.argv[1])
+target = sys.argv[1]
+if os.path.isdir(target):
+  for entry in os.scandir(target):
+    if entry.is_dir(follow_symlinks=False):
+      shutil.rmtree(entry.path)
+    else:
+      os.unlink(entry.path)