BUG: Prevent CMake cache variable from endlessly accumulating values

Previously, the CMake internal cache variable CTEST_CUSTOM_MEMCHECK_IGNORE
was appended to during each CMake configure step, but never reset. This
also caused the CMakeCache.txt file to grow endlessly. This commit fixes
this bug.

Additionally, a new macro 'itk_memcheck_ignore' is added, for setting tests
to be ignored by the MemCheck stage.

Change-Id: Id4cfd6ae4c2c040e7afa4133e9f41e6216afcb24
Merge topic 'Memcheck-Cache-Variable'

038da21 BUG: Prevent CMake cache variable from endlessly accumulating values