tests: extend test coverage of umovestr short read condition

* tests/umovestr3.c: New file.
* tests/umovestr3.test: New test.
* tests/.gitignore: Add umovestr3.
* tests/Makefile.am (check_PROGRAMS): Likewise.
(DECODER_TESTS): Add umovestr3.test.
diff --git a/tests/.gitignore b/tests/.gitignore
index 8a1e65a..2d9c4ab 100644
--- a/tests/.gitignore
+++ b/tests/.gitignore
@@ -230,6 +230,7 @@
 umount2
 umovestr
 umovestr2
+umovestr3
 uname
 unix-pair-send-recv
 unlink