Rebaseline test_small_js_flags. NFC (emscripten-core#23156)

Also convert to `check_expected_size_in_file` which allows this test
to be updated via `--rebaseline` 

This change shows the incremental progress we've been making on code
size recently!

test/browser/test_small_js_flags.js.size

@@ -0,0 +1 @@
+4369

test/common.py

@@ -1537,6 +1537,16 @@ def assertBinaryEqual(self, file1, file2):
     self.assertEqual(read_binary(file1),
                      read_binary(file2))
 
+  def check_expected_size_in_file(self, desc, filename, size):
+    if EMTEST_REBASELINE:
+      create_file(filename, f'{size}\n', absolute=True)
+    size_slack = 0.05
+    expected_size = int(read_file(filename).strip())
+    delta = size - expected_size
+    ratio = abs(delta) / float(expected_size)
+    print('  seen %s size: %d (expected: %d) (delta: %d), ratio to expected: %f' % (desc, size, expected_size, delta, ratio))
+    self.assertLess(ratio, size_slack)
+
   library_cache: Dict[str, Tuple[str, object]] = {}
 
   def get_build_dir(self):

test/test_browser.py

@@ -4204,7 +4204,9 @@ def test_small_js_flags(self):
     print('size:', size)
     # Note that this size includes test harness additions (for reporting the result, etc.).
     if not self.is_wasm64() and not self.is_2gb():
-      self.assertLess(abs(size - 4477), 100)
+      self.check_expected_size_in_file('js',
+                                       test_file('browser/test_small_js_flags.js.size'),
+                                       size)
 
   # Tests that it is possible to initialize and render WebGL content in a
   # pthread by using OffscreenCanvas.

test/test_other.py

@@ -8712,16 +8712,6 @@ def build(args):
     # adding --metrics should not affect code size
     self.assertEqual(base_size, os.path.getsize('a.out.wasm'))
 
-  def check_expected_size_in_file(self, desc, filename, size):
-    if common.EMTEST_REBASELINE:
-      create_file(filename, f'{size}\n', absolute=True)
-    size_slack = 0.05
-    expected_size = int(read_file(filename).strip())
-    delta = size - expected_size
-    ratio = abs(delta) / float(expected_size)
-    print('  seen %s size: %d (expected: %d) (delta: %d), ratio to expected: %f' % (desc, size, expected_size, delta, ratio))
-    self.assertLess(ratio, size_slack)
-
   @crossplatform
   def test_unoptimized_code_size(self):
     # We don't care too about unoptimized code size but we would like to keep it