tests: allow user to specify trash directory location

The tests generate a large amount of I/O activity creating
and destroying repositories and files. We can improve the
time it takes to run the test suite by creating trash
directories on filesystems with better performance
characteristic, even though we may not want the rest of the
git repository on those filesystems (e.g., because they are
not network connected, or because they are temporary
ramdisks).

For example, on a dual processor system:

  $ cd t && time make -j32
  real    1m51.562s
  user    0m59.260s
  sys     1m20.933s

  # /dev/shm is tmpfs
  $ cd t && time make -j32 GIT_TEST_OPTS="--root=/dev/shm"
  real    1m1.484s
  user    0m53.555s
  sys     1m5.264s

We almost halve the wall clock time, and we utilize the
dual processors much better.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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