Enable lint on test sources by default

[ZacSweers]

Apparently this is not enabled by default 🤦‍♂️

[github-actions]

Here are some suggestions to improve the code:

1. Simplify the SlackProperties class by using Kotlin's property delegation. This will eliminate the need for individual getter methods for each property. For example:

public class SlackProperties(private val project: Project) {
  public val lintBaselineFileName: String by stringProperty("slack.lint.baseline-file-name", "lint-baseline.xml")
  public val lintCheckTestSources: Boolean by booleanProperty("sgp.lint.checkTestSources", true)
  public val lintIgnoreTestSources: Boolean by booleanProperty("sgp.lint.ignoreTestSources", false)
  // ...
}

2. In the LintTasks object, instead of assigning properties one by one, you can use the apply function to set multiple properties at once. For example:

internal object LintTasks {
  // ...

  val lintOptions = LintOptions().apply {
    // ...
    ignoreTestSources = slackProperties.lintIgnoreTestSources
    checkTestSources = slackProperties.lintCheckTestSources
    // ...
  }

  // ...
}

3. Consider using a more descriptive name for the lintCheckTestSources and lintIgnoreTestSources properties in SlackProperties. The current names are a bit unclear and may cause confusion.

4. Add comments to explain the purpose and usage of the lintCheckTestSources and lintIgnoreTestSources properties in SlackProperties. This will make it easier for other developers to understand their purpose.

5. Consider using constants or enums instead of hardcoding string values for property keys. This will make the code more maintainable and less error-prone. For example:

private const val LINT_BASELINE_FILE_NAME_KEY = "slack.lint.baseline-file-name"
private const val LINT_CHECK_TEST_SOURCES_KEY = "sgp.lint.checkTestSources"
private const val LINT_IGNORE_TEST_SOURCES_KEY = "sgp.lint.ignoreTestSources"

public class SlackProperties(private val project: Project) {
  public val lintBaselineFileName: String by stringProperty(LINT_BASELINE_FILE_NAME_KEY, "lint-baseline.xml")
  public val lintCheckTestSources: Boolean by booleanProperty(LINT_CHECK_TEST_SOURCES_KEY, true)
  public val lintIgnoreTestSources: Boolean by booleanProperty(LINT_IGNORE_TEST_SOURCES_KEY, false)
  // ...
}

Overall, these suggestions aim to simplify and improve the readability of the code by leveraging Kotlin's language features and conventions.