t7006: guard cleanup with test_expect_success

t7006: guard cleanup with test_expect_success

Most of these tests are removing files, environment variables, and
configuration that might interfere outside the test.  Putting these
clean-up commands in the test (in the same spirit as v1.7.1-rc0~59,
2010-03-20) means that errors during setup will be caught quickly and
non-error text will be suppressed without -v.

While at it, apply some other minor fixes:

 - do not rely on the shell to export variables defined with the same
   command as a function call

 - avoid whitespace immediately after the > redirection operator, for
   consistency with the style of other tests

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>