Use require.progs with bc instead of require.files with /usr/bin/bc

This will make things more flexible if the program path changes in the future,
and the test in and of itself doesn't call /usr/bin/bc -- it just calls bc

MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division

bin/ls/tests/ls_tests.sh

@@ -535,7 +535,7 @@ atf_test_case h_flag
 h_flag_head()
 {
 	atf_set "descr" "Verify that -h prints out the humanized units for file sizes with ls -l"
-	atf_set "require.files" "/usr/bin/bc"
+	atf_set "require.progs" "bc"
 }
 
 h_flag_body()