make test: disable tests which require CXX >= 5.3 on rhel

same as commit d3e671e0dbb879d90f00bdee608ee0bb5f6357ae did for centos

Change-Id: If57765490d1ef41042a42db433b704af2f0c0ffd
Signed-off-by: Gabriel Ganne <gabriel.ganne@enea.com>
diff --git a/test/ext/Makefile b/test/ext/Makefile
index 0c92c69..542c6eb 100644
--- a/test/ext/Makefile
+++ b/test/ext/Makefile
@@ -4,7 +4,7 @@
 VOM_BINDIR = $(BR)/vom_test/
 VOM_BIN = $(addprefix $(VOM_BINDIR), vom_test)
 
-ifeq ($(filter centos,$(OS_ID)),$(OS_ID))
+ifeq ($(filter rhel centos,$(OS_ID)),$(OS_ID))
 VAPI_CPPBIN=
 endif