[1 of 2]: subscriber: add more parse_directives* tests

These test parse_directives() against strings that contain only a
legit log level name. The variants that submit the mixed case forms
are currently failing:

    $ cargo test --lib 'filter::env::directive::test::parse_directives_'
    ...
    failures:
        filter::env::directive::test::parse_directives_global_bare_warn_mixed
        filter::env::directive::test::parse_directives_ralith_mixed

    test result: FAILED. 12 passed; 2 failed; 0 ignored; 0 measured; 61 filtered out

Fix to come in a follow-up commit.

Co-authored-by: Eliza Weisman <eliza@buoyant.io>
 Signed-off-by: Alan D. Salewski <ads@salewski.email>

tracing-subscriber/src/filter/env/directive.rs

@@ -831,6 +831,32 @@ mod test {
         assert_eq!(dirs[1].in_span, None);
     }
 
+    #[test]
+    fn parse_directives_ralith_uc() {
+        let dirs = parse_directives("common=INFO,server=DEBUG");
+        assert_eq!(dirs.len(), 2, "\nparsed: {:#?}", dirs);
+        assert_eq!(dirs[0].target, Some("common".to_string()));
+        assert_eq!(dirs[0].level, LevelFilter::INFO);
+        assert_eq!(dirs[0].in_span, None);
+
+        assert_eq!(dirs[1].target, Some("server".to_string()));
+        assert_eq!(dirs[1].level, LevelFilter::DEBUG);
+        assert_eq!(dirs[1].in_span, None);
+    }
+
+    #[test]
+    fn parse_directives_ralith_mixed() {
+        let dirs = parse_directives("common=iNfo,server=dEbUg");
+        assert_eq!(dirs.len(), 2, "\nparsed: {:#?}", dirs);
+        assert_eq!(dirs[0].target, Some("common".to_string()));
+        assert_eq!(dirs[0].level, LevelFilter::INFO);
+        assert_eq!(dirs[0].in_span, None);
+
+        assert_eq!(dirs[1].target, Some("server".to_string()));
+        assert_eq!(dirs[1].level, LevelFilter::DEBUG);
+        assert_eq!(dirs[1].in_span, None);
+    }
+
     #[test]
     fn parse_directives_valid() {
         let dirs = parse_directives("crate1::mod1=error,crate1::mod2,crate2=debug,crate3=off");
@@ -1003,6 +1029,33 @@ mod test {
         assert_eq!(dirs[1].in_span, None);
     }
 
+    // helper function for tests below
+    fn test_parse_bare_level(directive_to_test: &str, level_expected: LevelFilter) {
+        let dirs = parse_directives(directive_to_test);
+        assert_eq!(dirs.len(), 1, "\ninput: \"{}\"; parsed: {:#?}", directive_to_test, dirs);
+        assert_eq!(dirs[0].target, None);
+        assert_eq!(dirs[0].level, level_expected);
+        assert_eq!(dirs[0].in_span, None);
+    }
+
+    #[test]
+    fn parse_directives_global_bare_warn_lc() {
+        // test parse_directives with no crate, in isolation, all lowercase
+        test_parse_bare_level("warn", LevelFilter::WARN);
+    }
+
+    #[test]
+    fn parse_directives_global_bare_warn_uc() {
+        // test parse_directives with no crate, in isolation, all uppercase
+        test_parse_bare_level("WARN", LevelFilter::WARN);
+    }
+
+    #[test]
+    fn parse_directives_global_bare_warn_mixed() {
+        // test parse_directives with no crate, in isolation, mixed case
+        test_parse_bare_level("wArN", LevelFilter::WARN);
+    }
+
     #[test]
     fn parse_directives_valid_with_spans() {
         let dirs = parse_directives("crate1::mod1[foo]=error,crate1::mod2[bar],crate2[baz]=debug");