Adjust test_oversample test case

Add -f to force generation of the background image at the desired
oversample resolution.  Our new behavior is to only send the oversampled
image to Tesseract while leaving the main page intact unless asked to
deskew, clean, etc.

tests/test_main.py

@@ -199,6 +199,7 @@ def test_override_metadata(spoof_tesseract_noop):
 def test_oversample(spoof_tesseract_cache, renderer):
     oversampled_pdf = check_ocrmypdf(
         'skew.pdf', 'test_oversample_%s.pdf' % renderer, '--oversample', '300',
+        '-f',
         '--pdf-renderer', renderer, env=spoof_tesseract_cache)
 
     pdfinfo = pdf_get_all_pageinfo(oversampled_pdf)