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.properties b/docs/knit.properties
index ab2508a..2028ecb 100644
--- a/docs/knit.properties
+++ b/docs/knit.properties
@@ -4,19 +4,7 @@
 
 knit.package=kotlinx.coroutines.guide
 knit.dir=../kotlinx-coroutines-core/jvm/test/guide/
-knit.pattern=example-[a-zA-Z0-9-]+-##\\.kt
-knit.include=knit.code.include
 
 test.package=kotlinx.coroutines.guide.test
 test.dir=../kotlinx-coroutines-core/jvm/test/guide/test/
-test.template=knit.test.template
 
-# Various test validation modes and their corresponding methods from TestUtil
-test.mode.=verifyLines
-test.mode.STARTS_WITH=verifyLinesStartWith
-test.mode.ARBITRARY_TIME=verifyLinesArbitraryTime
-test.mode.FLEXIBLE_TIME=verifyLinesFlexibleTime
-test.mode.FLEXIBLE_THREAD=verifyLinesFlexibleThread
-test.mode.LINES_START_UNORDERED=verifyLinesStartUnordered
-test.mode.LINES_START=verifyLinesStart
-test.mode.EXCEPTION=verifyExceptions
\ No newline at end of file
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} {