tests: add shmxt.test

* tests/shmxt.c: New file.
* tests/shmxt.test: New test.
* tests/.gitignore: Add shmxt.
* tests/Makefile.am (check_PROGRAMS): Likewise.
(DECODER_TESTS): Add shmxt.test.
diff --git a/tests/Makefile.am b/tests/Makefile.am
index f31b600..5069c19 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -193,6 +193,7 @@
 	sendfile64 \
 	set_ptracer_any \
 	sethostname \
+	shmxt \
 	sigaction \
 	sigaltstack \
 	signalfd \
@@ -399,6 +400,7 @@
 	sendfile.test \
 	sendfile64.test \
 	sethostname.test \
+	shmxt.test \
 	sigaction.test \
 	sigaltstack.test \
 	signalfd.test \