fix incorrect porting from % formatting to .format method

(var,) is the correct way to pass values to a percent formatted string,
but not to .format so we would end up printing something like:

unexpected input at line (4,)

Upgrade to an f-string and insert the correct value correctly.

mesonbuild/mtest.py

@@ -430,7 +430,7 @@ class TAPParser:
             if not line:
                 return
 
-            yield self.Error('unexpected input at line {}'.format((self.lineno,)))
+            yield self.Error(f'unexpected input at line {self.lineno}')
         else:
             # end of file
             if self.state == self._YAML: