Improve tests.

[jhlegarreta]

The existing tests should be improved:

• The files CoherenceEnhancingDiffusionCommandLine.h and LinearAnisotropicDiffusionCommandLine.h are not actually needed for the module functionality, and only serve for testing purposes. They should be moved into the test directory, and tests should be refactored to follow ITK testing conventions (i.e. their contents should be in a *.cxx implementation file).
• The ITK testing macros should be used (e.g. TRY_EXPECT_NO_EXCEPTION, EXERCISE_BASIC_OBJECT_METHODS, etc.) to avoid boilerplate code.
• The tests should run as long as possible and return the status code at the end (i.e. use a testStatus in and change its value as tests fail: requires changing the test design).
• Improve the argument check message.
• Remove the unnecessary message prints.
• The coding style should conform to the ITK coding style (Appendix Three).