"Fix" tests with differing logical/physical paths

"Fix" tests with differing logical/physical paths

Explicitly "cd $(pwd -P)" before testing to work around problems
relating to unresolved questions about the correct behavior of drecurse
when the logical and physical paths differ.

The problem can be reproduced by entering a bup source directory via a
symlink and running "make check":

  ln -s bup-real bup
  cd bup
  make check

Thanks to Brandon Smith and Greg Troxel for reporting the
problem (again) and proposing slightly different solutions.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>