Add tests for a couple of fixed issues found by libFuzzer

From:
https://github.com/behdad/harfbuzz/issues/139#issuecomment-147616887
https://github.com/behdad/harfbuzz/issues/139#issuecomment-148289957

test/shaping/fonts/sha1sum/MANIFEST

@@ -6,11 +6,13 @@
 270b89df543a7e48e206a2d830c0e10e5265c630.ttf
 298c9e1d955f10f6f72c6915c3c6ff9bf9695cec.ttf
 37033cc5cf37bb223d7355153016b6ccece93b28.ttf
+375d6ae32a3cbe52fbf81a4e5777e3377675d5a3.ttf
 43ef465752be9af900745f72fe29cb853a1401a5.ttf
 4cce528e99f600ed9c25a2b69e32eb94a03b4ae8.ttf
 5028afb650b1bb718ed2131e872fbcce57828fff.ttf
 57a9d9f83020155cbb1d2be1f43d82388cbecc88.ttf
 5a5daf5eb5a4db77a2baa3ad9c7a6ed6e0655fa8.ttf
+641bd9db850193064d17575053ae2bf8ec149ddc.ttf
 6466d38c62e73a39202435a4f73bf5d6acbb73c0.ttf
 757ebd573617a24aa9dfbf0b885c54875c6fe06b.ttf
 7e14e7883ed152baa158b80e207b66114c823a8b.ttf

test/shaping/tests/fuzzed.tests

 fonts/sha1sum/1a6f1687b7a221f9f2c834b0b360d3c8463b6daf.ttf::U+0041:[gid0=0+1000]
 fonts/sha1sum/5a5daf5eb5a4db77a2baa3ad9c7a6ed6e0655fa8.ttf::U+0041:[gid0=0+1000]
 fonts/sha1sum/0509e80afb379d16560e9e47bdd7d888bebdebc6.ttf::U+0041:[gid0=0+1000]
+fonts/sha1sum/641bd9db850193064d17575053ae2bf8ec149ddc.ttf::U+0041:[gid0=0+1000]
+fonts/sha1sum/375d6ae32a3cbe52fbf81a4e5777e3377675d5a3.ttf::U+0041:[gid0=0+2048]