btrfs-progs: test/common: Enhance prepare_test_dev to reset device size

So prepare_test_dev() can be called several times in one test case, to
test different device sizes.

Signed-off-by: Qu Wenruo <wqu@suse.com>
[ switch to [ ] ]
Signed-off-by: David Sterba <dsterba@suse.com>

tests/common

@@ -441,8 +441,12 @@ prepare_test_dev()
 	# num[K/M/G/T...]
 	local size="$1"
 
-	[[ "$TEST_DEV" ]] && return
 	[[ "$size" ]] || size='2G'
+	# Still truncate it to new size
+	if [ -n "$TEST_DEV" ]; then
+		truncate -s "$size" "$TEST_DEV"
+		return;
+	fi
 
 	echo "\$TEST_DEV not given, use $TOP/test/test.img as fallback" >> \
 		"$RESULTS"