Created an initial test case for ComparisonFailures

Partly Taken from the old junit.framework.ComparisonCompactorTest.

Also added test cases for strings with common suffix/prefix.
Switching to a Parameterized test class.

Added long pre/postfix test cases.

All relevant test cases for message compacting included.

Included tests cases from (old) ComparisonCompactorTest
that are still relevant for current ComparisonFailure class.

Added empty string boundary cases.

Branch coverage back to normal.
Refactoring ensuring fSuffix is equal to length of suffix.

While fPrefix was equal to length of prefix, in old code
fSuffix was equal to length of suffix + 1.

Made the two consistent, resulting in simplification of suffix
computation (removed "+ 1" four times, added "- 1" once).

Found inconsistency through branch coverage analysis, which
suggested that condition "fSuffix > 0" could never be false.
Reworded postfix to suffix.
Merge pull request #675 from avandeursen/compactfailuretest

Test class for String Comparison Compactors, and off-by-one refactoring of compactor.