Fix junit tests under windows when the platform is explicitly changed

Fix junit tests under windows when the platform is explicitly changed

SystemReader used a chached ObjectChecker which was instantiated only
once. But in case of unit tests where we can change the platform
dynamically (e.g. MockSystemReader.setWindows()) this is wrong and
caused DirCacheCheckoutMaliciousPathTest.
testMaliciousAbsoluteCurDrivePathWindowsOnUnix() to fail. This change
allows
user of SystemReader to force the creation of a new ObjectChecker.
MockSystemReader.setWindows() and .setUnix() make use of this feature.

Change-Id: I87458d1dc63c1f5c18979f972b1c1f0d670a9ed8