Fix self-comparison and assert typos in TestSemantics (#191938)

This replaces #185975, which the stale bot auto-closed on Jul 1. The
requested tests had been pushed on May 21 (commit "Add semantics tester
regression tests"), but the `waiting for response` label was never
cleared, so the timer kept running. #185975 could not be reopened, hence
this PR.

@chunhtai you'd reviewed #185975 and said it lgtm pending a test. That
test is here — three regression tests, one per fix:

- `Semantics tester compares controlsNodes`
- `Semantics tester compares attributed string locales`
- `includesNodeWith accepts minValue and maxValue`

## The bugs

Rebased onto current master, where all three are still present:

- **`semantics_tester.dart:511`** — `controlsNodes != controlsNodes`
compares the field to itself, so it is always `false` and the
`setEquals` check is unreachable. The `controlsNodes` expectation is
silently never enforced. Fixed to `controlsNodes != null`.
- **`semantics_tester.dart:715`** — the locale comparison reads
`(second[i] as LocaleStringAttribute).locale != (second[i] as
LocaleStringAttribute).locale`, comparing `second` against itself rather
than against `first`. Always `false`, so mismatched locales compare
equal. Fixed to read `first[i]` on the left.
- **`semantics_tester.dart:1184`** — the `minValue != null || maxValue
!= null` checks sit in `assert()`'s *message* slot rather than its
condition, so they never gate anything and `includesNodeWith(minValue:
...)` alone trips the assert. Folded into the condition, with a
descriptive message added.

## Testing

`flutter test packages/flutter/test/widgets/semantics_tester_test.dart`
— 7/7 passing locally against current master.

The competing PR #185921 was also closed without merging, so there is no
longer any overlap. PTAL when you have a moment.