Add is_fedora and exit_distro_not_supported functions

Between is_fedora, is_ubuntu and is_suse, we can make the code a bit
simpler to read. We also use exit_distro_not_supported to identify
places where we need implementation details for new distros.

As "/sbin/service --skip-redirect" is Fedora-specific, guard this with a
is_fedora test too.

Change-Id: Ic77c0697ed9be0dbb5df8e73da93463e76025f0c
diff --git a/tests/functions.sh b/tests/functions.sh
index d2cc5c4..be48729 100755
--- a/tests/functions.sh
+++ b/tests/functions.sh
@@ -260,9 +260,11 @@
 if [[ "$os_PACKAGE" = "deb" ]]; then
     is_package_installed dpkg
     VAL=$?
-else
+elif [[ "$os_PACKAGE" = "rpm" ]]; then
     is_package_installed rpm
     VAL=$?
+else
+    VAL=1
 fi
 if [[ "$VAL" -eq 0 ]]; then
     echo "OK"
@@ -273,9 +275,11 @@
 if [[ "$os_PACKAGE" = "deb" ]]; then
     is_package_installed dpkg bash
     VAL=$?
-else
+elif [[ "$os_PACKAGE" = "rpm" ]]; then
     is_package_installed rpm bash
     VAL=$?
+else
+    VAL=1
 fi
 if [[ "$VAL" -eq 0 ]]; then
     echo "OK"