Only show images during tests when requested via project property (#2177)

## Description

Revision of #2164. Instead of only running headless tests during a
build, now only the images are disabled. To reenable showing images, the
`enableTestUi` project property needs to be passed to gradle.

```bash
./gradlew test -PenableTestUi
```

## Meta

Merge checklist:
- [X] Pull Request title is [short, imperative
summary](https://cbea.ms/git-commit/) of proposed changes
- [X] The description documents the _what_ and _why_
- [X] This PR has been
[linted](https://docs.photonvision.org/en/latest/docs/contributing/linting.html).
- [x] If this PR changes behavior or adds a feature, user documentation
is updated
- [ ] If this PR touches photon-serde, all messages have been
regenerated and hashes have not changed unexpectedly
- [ ] If this PR touches configuration, this is backwards compatible
with settings back to v2025.3.2
- [ ] If this PR touches pipeline settings or anything related to data
exchange, the frontend typing is updated
- [ ] If this PR addresses a bug, a regression test for it is added

docs/source/docs/contributing/building-photon.md

@@ -177,6 +177,29 @@ With the VSCode [Extension Pack for Java](https://marketplace.visualstudio.com/i
 
 To correctly run PhotonVision tests this way, you must [delegate the tests to Gradle](https://code.visualstudio.com/docs/java/java-build#_delegate-tests-to-gradle). Debugging tests like this will [**not** currently](https://github.com/microsoft/build-server-for-gradle/issues/119) collect outputs.
 
+### Running Tests With UI
+
+By default, tests are run with UI disabled so they are not obtrusive during a build. All tests should be useful when the UI is disabled. However, if a particular test would benefit from having UI access (i.e. for debugging info), the UI can be enabled by passing the `enableTestUi` project property to Gradle. This will run all tests by default, but the Gradle `--tests` option can be used to [filter for specific tests](https://docs.gradle.org/current/userguide/java_testing.html#test_filtering).
+
+```{eval-rst}
+.. tab-set::
+
+   .. tab-item:: Linux
+      :sync: linux
+
+      ``./gradlew test -PenableTestUi``
+
+   .. tab-item:: macOS
+      :sync: macos
+
+      ``./gradlew test -PenableTestUi``
+
+   .. tab-item:: Windows (cmd)
+      :sync: windows
+
+      ``gradlew test -PenableTestUi``
+```
+
 ### Debugging PhotonVision Running Locally
 
 Unit tests can instead be debugged through the ``test`` Gradle task for a specific subproject in VSCode, found in the Gradle tab: