[test] Prefer shutil.copy over shutil.copyfile. NFC (#22652)

The former is more flexible as it allows the target to be a directory
name.

Also remove some unnecessary use of `self.in_dir`.

Author: sbc100

test/common.py

@@ -2192,7 +2192,7 @@ def make_reftest(self, expected, manually_trigger=False):
     # make sure the pngs used here have no color correction, using e.g.
     #   pngcrush -rem gAMA -rem cHRM -rem iCCP -rem sRGB infile outfile
     reporting = read_file(test_file('browser_reporting.js'))
-    shutil.copyfile(expected, 'expected.png')
+    shutil.copy(expected, 'expected.png')
     create_file('reftest.js', '''
       function doReftest() {
         if (doReftest.done) return;