Line numbers start at 1

Bumps the line number for the singleton warning. When the singleton
occurs on the same line at the term starts, the line number is correct:

    foo(X).

However it stills mis-reports this line number as 1 instead of 5:

    foo(1) :-
        true,
        true,
        true,
        Y.

See issue #1356.

tests/scryer/main.rs

@@ -22,7 +22,6 @@ fn cli_tests() {
     trycmd::TestCases::new()
         .default_bin_name("scryer-prolog")
         .case("tests/scryer/cli/issues/*.toml")
-        .skip("tests/scryer/cli/issues/singleton_warning.toml") // wrong line number
         .case("tests/scryer/cli/src_tests/*.toml")
         .case("tests/scryer/cli/src_tests/*.md");
 }