More fixes towards invalid tabs?

parser/src/scanner.rs

@@ -1915,7 +1915,7 @@ impl<T: Iterator<Item = char>> Scanner<T> {
         self.skip();
         if self.look_ch() == '\t'
             && !self.skip_ws_to_eol(SkipTabs::Yes).has_valid_yaml_ws()
-            && self.ch() == '-'
+            && (self.ch() == '-' || is_alpha(self.ch()))
         {
             return Err(ScanError::new(
                 self.mark,

parser/tests/yaml-test-suite.rs

@@ -301,7 +301,6 @@ static EXPECTED_FAILURES: &[&str] = &[
     // TAB as start of plain scalar instead of whitespace
     // TABs in whitespace-only lines
     // TABs after marker ? or : (space required?)
-    "Y79Y-09",
     // Other TABs
     "DK95-01", // in double-quoted scalar
     // Empty key in flow mappings