Add test to validate failure of spot in this test case.

When dumping a format 2 PosRule from a lookup Type 7, spot was writing past the end of a fixed length string buffer and destroying the stack.

Fix test to validate failure of spot in this test case - did not fail with crash because spot reported that output file was bad in Appveyor run.

Fix test to validate failure of spot in this test case - on Windows, needed to overrun by the buffer by more than 1K.

Fix text buffer overrun by writing it out whenever the buffer gets close to its limit,

Change test to validate success of spot in this test case.

Fix test to validate success of spot in this test case: correct typo, and that the tests requires success rather than failure.

Fix spacing in message string. GPOS.c

Updated expected output file to match previous
six spacing in message string. GPOS.c.