Use '-Wl,-bexpall' when linking unittest binaries on AIX

Some unittests provide dummy versions of some standard functions
and we need to make sure these take precedence over the real
standard functions in libraries that we are linking with.

Ticket: ENT-4487
Changelog: None

tests/unit/Makefile.am

@@ -404,7 +404,10 @@ endif
 if AIX
 
 check_PROGRAMS += aix_process_test
-
+# We need to use -Wl,-bexpall when linking tests binaries on AIX
+# because they provide dummy versions of some standard functions.
+set_domainname_test_LDFLAGS = -Wl,-bexpall
+evalfunction_test_LDFLAGS = -Wl,-bexpall
 aix_process_test_SOURCES = aix_process_test.c \
 	../../libpromises/process_unix.c \
 	../../libpromises/process_aix.c \