tests: use $TEST_DIRECTORY to refer to the t/ directory

Many test scripts assumed that they will start in a 'trash' subdirectory
that is a single level down from the t/ directory, and referred to their
test vector files by asking for files like "../t9999/expect".  This will
break if we move the 'trash' subdirectory elsewhere.

To solve this, we earlier introduced "$TEST_DIRECTORY" so that they can
refer to t/ directory reliably.  This finally makes all the tests use
it to refer to the outside environment.

With this patch, and a one-liner not included here (because it would
contradict with what Dscho really wants to do):

| diff --git a/t/test-lib.sh b/t/test-lib.sh
| index 70ea7e0..60e69e4 100644
| --- a/t/test-lib.sh
| +++ b/t/test-lib.sh
| @@ -485,7 +485,7 @@ fi
|  . ../GIT-BUILD-OPTIONS
|
|  # Test repository
| -test="trash directory"
| +test="trash directory/another level/yet another"
|  rm -fr "$test" || {
|         trap - exit
|         echo >&5 "FATAL: Cannot prepare test area"

all the tests still pass, but we would want extra sets of eyeballs on this
type of change to really make sure.

[jc: with help from Stephan Beyer on http-push tests I do not run myself;
 credits for locating silly quoting errors go to Olivier Marin.]

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4012-diff-binary.sh

@@ -12,7 +12,7 @@ test_expect_success 'prepare repository' \
 	'echo AIT >a && echo BIT >b && echo CIT >c && echo DIT >d &&
 	 git update-index --add a b c d &&
 	 echo git >a &&
-	 cat ../test4012.png >b &&
+	 cat "$TEST_DIRECTORY"/test4012.png >b &&
 	 echo git >c &&
 	 cat b b >d'