Update Flow.sample KDoc example timings, add tests (#2259)

* All Flow.debounce/sample KDoc example code snippets are automatically tested with Knit.
* Flow.sample timings are made larger, so that they produce an expected output when run under the real time, too.

Fixes #2243
diff --git a/docs/knit.test.template b/docs/knit.test.template
index a912555..727493c 100644
--- a/docs/knit.test.template
+++ b/docs/knit.test.template
@@ -5,6 +5,7 @@
 // This file was automatically generated from ${file.name} by Knit tool. Do not edit.
 package ${test.package}
 
+import kotlinx.coroutines.knit.*
 import org.junit.Test
 
 class ${test.name} {