xattr: skip unittest on unsupported platforms

The stripcredentials unittest fails to compile on platforms without
xattr support, for example the Solaris member in the buildfarm which
fails with the following:

  CC unit1621-unit1621.o
  CC ../libtest/unit1621-first.o
  CCLD unit1621
  Undefined first referenced
  symbol in file
  stripcredentials unit1621-unit1621.o
  goto problem 2
  ld: fatal: symbol referencing errors. No output written to .libs/unit1621
  collect2: error: ld returned 1 exit status
  gmake[2]: *** [Makefile:996: unit1621] Error 1

Fix by excluding the test on such platforms by using the reverse
logic from where stripcredentials() is defined.

Closes #3759
Reviewed-by: Daniel Stenberg <daniel@haxx.se>