btrfs-progs: tests: Introduce init_env to initialize common env variant

btrfs-progs: tests: Introduce init_env to initialize common env variant

For example, $TEST_DIR is common used in severial tests, and have
duplicated code for initialize.

These duplicated code not only benifits harddisk vendor, but have
inconsistent details, as:
  convert-tests.sh: lack of mkdir
  fsck-tests/012-leaf-corruption/test.sh: unnecessary mkdir
  fsck-tests/013-extent-tree-rebuild/test.sh: unnecessary init
  misc-tests/XXX ...
And severial error message:
  _fail "unable to create mount point on $TEST_MNT"
  _fail "failed to create mount point"
  ...

This patch move initizlizaton of $TEST_DIR to common init_env(),
to avoid above problem, and init_env() can be used to add more
things in future.

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>