test: Differentiate ability for gnu utils to handle symlinks from git

t7800 failed under symlink enabled msysgit without symlink enabled perl/msys

Signed-off-by: Michael Geddes <michael@frog.wheelycreek.net>

Author: frogonwheels

t/t7800-difftool.sh

@@ -371,7 +371,7 @@ do
 done >actual
 EOF
 
-test_expect_success PERL,SYMLINKS 'difftool --dir-diff --symlink without unstaged changes' '
+test_expect_success PERL,SYMLINKS_SH 'difftool --dir-diff --symlink without unstaged changes' '
 	cat >expect <<-EOF &&
 	file
 	$(pwd)/file

t/test-lib.sh

@@ -958,6 +958,12 @@ test_lazy_prereq SYMLINKS '
 	touch x
 	ln -s x y && test -h y
 '
+test_lazy_prereq SYMLINKS_SH '
+	# test whether the filesystem supports symbolic links
+	touch x
+	# Then check it is supported by the shell (not using overridden test)
+	ln -s x y && builtin test -h y
+'
 
 test_lazy_prereq FILEMODE '
 	test "$(git config --bool core.filemode)" = true