[mustache_template] Fix auto-generated specification tests, run with dart test (#11056)

This PR reorganizes the tests in mustache_template such that CI can run them, per the second option proposed in flutter/flutter#174721. The changes include a script to pull mustache specifications from the mustache repository and note the time/date and commit hash of that action, along with documentation of the above and the new test structure laid out below.

The former `all.dart` (which called three `main` functions in three test files) is now `mustache_test.dart`, and the former `mustache_test.dart` (which contained handwritten tests) is now `feature_test.dart`. The `mustache_specs.dart` file is now solely responsible for generating the tests in `mustache_test.dart` from the mustache specs, and no longer has a `main` of its own.

As this implementation doesn't support two of the optional mustache specs (inheritance and dynamic names, which were added to the mustache repo in the early 2020s, while the original mustache_template package was written in the mid-2010s), this PR also adds an UNSUPPORTED_SPECS constant in `mustache_test.dart` that skips generation of tests from those spec files.

(Separately, I'm working on an implementation of the inheritance spec, so it seemed better to pull all the specification files and selectively disable the unsupported ones.)

## Pre-Review Checklist

### Exemptions
- Version change exemption: PR affects tests only.
- CHANGELOG exemption: PR affects tests only.

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Author: jslater89

third_party/packages/mustache_template/.gitignore

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+test/spec

third_party/packages/mustache_template/test/README.md

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+# Test Structure
+
+## Specifications
+
+`mustache_test.dart` and `mustache_specs.dart` generate tests for all mustache specifications,
+except for those disabled by `mustache_test.dart`'s `unsupportedSpecs` constant.
+`dart test mustache_test.dart` runs all of the generated tests.
+
+Each generated specification file contains the mustache commit hash from which it was generated,
+and the date when it was generated.
+
+### Updating specifications
+
+From the package root, run `dart run tool/download_spec.dart` to pull new definitions from the
+mustache repository into `test/specs/`.
+
+## Features
+
+Standalone or handwritten tests regarding template output go in `feature_test.dart`.
+
+## Parser
+
+Tests for the template language parser go in `parser_test.dart`.