tests: Add test_serial

Uses pipes to connect two instances (A and B) of the serial binding and
passes a message exceeding the BTU from A to B.

Increases test converage for libmctp as a whole from ~60% (lines and
functions) to ~75% (lines and functions).

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: Iaad6f265528799148c23db9365ebf9fa748e50ee
diff --git a/Makefile.am b/Makefile.am
index e967280..3994ff0 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -35,7 +35,7 @@
 TESTS = $(check_PROGRAMS)
 
 check_PROGRAMS = tests/test_eid tests/test_seq tests/test_bridge \
-		 tests/test_astlpc
+		 tests/test_astlpc tests/test_serial
 # We set a global LDADD here, as there's no way to specify it for all
 # tests. This means other targets' LDADDs need to be overridden.
 LDADD = tests/libtest-utils.a libmctp.la