BUG: Regenerate HeaderTests' after .hxx files are removed.

The HeaderTests were not regenerated appropriately when only .hxx files
were moved around because the .hxx files were not included in the MD5 hash used
to keep track of module header contents.

Suggested-by: Bill Hoffman <bill.hoffman@kitware.com>

Change-Id: I1593cbedd1d221e191ee9c7d1df7085d6b5b78c7
Merge topic 'HeaderTest'

9b640a5 BUG: Regenerate HeaderTests' after .hxx files are removed.