Make test built of 'libbet' reproducible with old Pillow versions (#1385)

The Python Pillow image library gives different output for
`Image.getcolors()` in version 9 than version 10. This causes
libbet's SGB border to be built differently, giving a different
ROM hash and technically failing the test script.

This patches libbet to only keep the known-good colors. It also
adapts the coverage script to run all the tests, including
building the third-party projects.

test/patches/libbet.patch

@@ -24,3 +24,16 @@ index 6513518..ba14638 100644
  	$(PY) $^ -o $@
  
  # Graphics conversion
+diff --git a/tools/makeborder.py b/tools/makeborder.py
+index b9fa3f1..dfe5789 100755
+--- a/tools/makeborder.py
++++ b/tools/makeborder.py
+@@ -88,7 +88,7 @@ def main(argv=None):
+     else:
+         iutmrows = b''.join(pb16.pb16(b''.join(utmrows)))
+ 
+-    palette = im.getpalette()[:48]
++    palette = im.getpalette()[:12]
+     palette.extend(bytes(48 - len(palette)))
+     snespalette = bytearray()
+     for i in range(0, 48, 3):