test(library): add util for listing hosted runner labels

github-workflows-kt/src/test/kotlin/io/github/typesafegithub/workflows/domain/GithubHostedRunnerLabelsListing.kt

@@ -0,0 +1,27 @@
+package io.github.typesafegithub.workflows.domain
+
+import io.ktor.client.HttpClient
+import io.ktor.client.request.get
+import io.ktor.client.statement.bodyAsText
+
+/**
+ * Lists values allowed to be set in "runs-on" for a job, for GitHub-hosted runners.
+ */
+suspend fun listRunnerLabels(): List<String> {
+    val html = HttpClient().get(GITHUB_RUNNER_LABELS_DOCS_URL).bodyAsText()
+    return parse(html)
+}
+
+private const val GITHUB_RUNNER_LABELS_DOCS_URL: String =
+    "https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions"
+
+private fun parse(html: String): List<String> =
+    RUNNER_LABEL_REGEX
+        .findAll(html)
+        .map { it.groupValues[1] }
+        .toList()
+
+private val RUNNER_LABEL_REGEX =
+    Regex(
+        """<code><a href="[^"]*github\.com/actions/(?:partner-)?runner-images/[^"]*">([^<]+)</a></code>""",
+    )