FEDX-1987: Only generate backwards-compatible output when necessary

[evanweible-wf]

FEDX-1987

In v3, we moved the output of the browser-aggregation test config from test/dart_test.browser_aggregate.yaml to dart_test.browser_aggregate.yaml (in the root). For backwards-compatibility, we continued to output to the deprecated location, too. Most consumers should already be including the dart_test.browser_aggregate.yaml in the root instead of the copy from the test/ directory, but if they aren't, it should be trivial to detect by reading dart_test.yaml where it would need to be included.

This PR makes that change: when test/dart_test.browser_aggregate.yaml is found in dart_test.yaml, the builder will output that file. Otherwise, it will not, and only the root dart_test.browser_aggregate.yaml will be generated.

[evanweible-wf]

The example tests are timing out on stable and beta because the dart run build_runner test process is not exiting even though the child test process has exited.. not sure why. I opened an issue here: dart-lang/build#3770

[evanweible-wf]

This is passing CI now with the workarounds in place.

[todbachman-wf]

Suggestion: What would you think about making this an info or warning log that includes a suggestion to update the dart_test.yaml file to reference the new file?

[todbachman-wf]

QA +1. I pulled this into wdesk_sdk and verified that the backwards compatible file is no longer created when running the tests in browser aggregation mode.

[todbachman-wf]

+10

[robbecker-wf]

@Workiva/release-management-p

[rmconsole-wf]

+1 from RM

[patkujawa-wf]

Something recently started causing CI failures like https://github.com/Workiva/tasker-ui/actions/runs/11962147002 due to

[INFO] build_web_compilers:entrypoint on test/templates/test_template.browser_aggregate_test.dart.browser_test.dart:Info: Compiling without sound null safety!
Dart 3 will only support sound null safety, see https://dart.dev/null-safety
org-dartlang-app:///packages/test_api/src/frontend/fake.dart:49:11:
Hint: This 'noSuchMethod' implementation is guaranteed to throw an exception. The generated code will be smaller if it is rewritten.
Error on line 8, column 10 of dart_test.yaml: Cannot open file, path = './test/dart_test.browser_aggregate.yaml' (OS Error: No such file or directory, errno = 2)
╷
8 │ include: test/dart_test.browser_aggregate.yaml
│ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
╵
Rewrite to 'noSuchMethod(Invocation i) => super.noSuchMethod(i);'.
dynamic noSuchMethod(Invocation invocation) {
^
org-dartlang-app:///packages/mocktail/src/mocktail.dart:111:11:
Hint: Overriding 'noSuchMethod' causes the compiler to generate more code and prevents the compiler from doing some optimizations.
Consider removing this 'noSuchMethod' implementation.
dynamic noSuchMethod(Invocation invocation) {
^
Compiled 71,268,909 characters Dart to 8,423,141 characters JavaScript in 169 seconds using 3068.316 MB of memory
Dart file org-dartlang-app:///test/templates/test_template.browser_aggregate_test.dart.browser_test.dart compiled to JavaScript: test/templates/test_template.browser_aggregate_test.dart.browser_test.dart.js