mtest: ignore invalid input

TAP version 14 introduced subtests, that are supposedly backward compatible
because "TAP13 specifies that non-TAP output should be ignored".  Meson
reported TAP syntax errors based on behavior of "prove" at the time,
but it seems that now "prove" has become a lot more lenient; it even
accepts the following completely bogus input just fine:

---
ok 1
    ok 2
x
1..1
---

So do the same and make Meson's parser accept invalid TAP input silently.

Fixes: #10032

mesonbuild/mtest.py

@@ -426,11 +426,6 @@ class TAPParser:
                 else:
                     yield self.Version(version=self.version)
                 return
-
-            if not line:
-                return
-
-            yield self.Error(f'unexpected input at line {self.lineno}')
         else:
             # end of file
             if self.state == self._YAML:

unittests/taptests.py

@@ -255,7 +255,6 @@ class TAPParserTests(unittest.TestCase):
     def test_unexpected(self):
         events = self.parse_tap('1..1\ninvalid\nok 1')
         self.assert_plan(events, num_tests=1, late=False)
-        self.assert_error(events)
         self.assert_test(events, number=1, name='', result=TestResult.OK)
         self.assert_last(events)