test-lib: Simplify test counting.

Since the test case counter was incremented very late, there were a few
users of the counter had to do their own incrementing. Now we increment it
early and simplify these users.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>

t/t4013-diff-various.sh

@@ -101,8 +101,7 @@ do
 	'' | '#'*) continue ;;
 	esac
 	test=`echo "$cmd" | sed -e 's|[/ ][/ ]*|_|g'`
-	cnt=`expr $test_count + 1`
-	pfx=`printf "%04d" $cnt`
+	pfx=`printf "%04d" $test_count`
 	expect="$TEST_DIRECTORY/t4013/diff.$test"
 	actual="$pfx-diff.$test"