chimera: fix unit tests

Two unit tests are racy because they assume at least 1 ms has elapsed
between two calls to stat.  This patch has the testing thread sleep
for a millisecond to ensure this is true.

The patch also alters how the directory listing helper handles invalid
situations, with it throwing an exception rather than logging an error.
This allows the unit-test that checks this behaviour to do so without
logging the error.

Target: master
Request: 2.10
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6

Conflicts:
	modules/chimera/src/main/java/org/dcache/chimera/DirectoryStreamHelper.java
	modules/chimera/src/test/java/org/dcache/chimera/DirectoryStreamHelperTest.java