RFC: Test hook for nss_files testing

[Florian Weimer]

The attached patch adds a test hook which allows us to select different
files (not those under /etc) for testing nss_files inside the build
tree.  It contains a regression test for bug 17363 as an example.

If the direction is acceptable, I'll add further tests.

This change increases the size of nss_files slightly (on x86_64):

Old:

   text    data     bss     dec     hex filename
  40895     992   26040   67927   10957 nss/libnss_files.so.2

New:

   text    data     bss     dec     hex filename
  41426    1088   26040   68554   10bca nss/libnss_files.so.2

If this is a problem, I can make additional cleanups which reduce the
DSO size to the previous level.

There are also a few additional relocations (for the string constants),
but they should not matter.

Florian
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