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.

contrib/coverage.bash

@@ -5,11 +5,10 @@ set -e
 make coverage -j
 
 # Run the tests
-for dir in asm link fix gfx; do
-	pushd test/$dir
-	./test.sh
-	popd
-done
+pushd test
+./fetch-test-deps.sh
+./run-tests.sh
+popd
 
 # Generate coverage logs
 gcov src/**/*.cpp