MAC/do: Tests: Quote the source directory

In a standard test suite installation, this is not necessary, but be
bullet-proof to custom ones, however improbable.

Reviewed by:    bapt
MFC after:      3 days
Sponsored by:   The FreeBSD Foundation
Pull Request:   https://ron-dev.freebsd.org/FreeBSD/src/pulls/38

tests/sys/mac/do/invalid_configs.sh

@@ -75,7 +75,7 @@ rules_wrong_separator_body()
 
 atf_init_test_cases()
 {
-    . $(atf_get_srcdir)/common.sh
+    . "$(atf_get_srcdir)"/common.sh
 
     atf_add_test_case rule_no_target_part
     atf_add_test_case rule_no_match_part

tests/sys/mac/do/valid_configs.sh

@@ -120,7 +120,7 @@ gid= 1001 >gid =5"
 
 atf_init_test_cases()
 {
-    . $(atf_get_srcdir)/common.sh
+    . "$(atf_get_srcdir)"/common.sh
 
     atf_add_test_case rule_uid_to_any
     atf_add_test_case rule_uid_to_uid