Improve handling of stacktrace tests on JDK 8

Before, the tests only knew that the `park` native method was moved
to `jdk.internal.misc.Unsafe` from `sun.misc.Unsafe`. However, in
JDK 11, there is no `sun.misc.Unsafe` at all, it seems that it
has been moved completely.

This change is beneficial for this feature set because BlockHound
puts its `$$BlockHound$$_park` method to `sun.misc.Unsafe` or to
`jdk.internal.misc.Unsafe` depending on the JDK version. Also,
judging by BlockHound's code
https://github.com/reactor/BlockHound/blob/091d7b139479b1c41eea59baa23389d673fdf73b/agent/src/main/java/reactor/blockhound/BlockHound.java#L177-L187,
`forkAndExec` had been moved, too, but it is not used in tests.

Author: dkhalanskyjb

kotlinx-coroutines-debug/test/StracktraceUtils.kt

@@ -13,7 +13,7 @@ public fun String.trimStackTrace(): String =
         .replace(Regex("#[0-9]+"), "")
         .replace(Regex("(?<=\tat )[^\n]*/"), "")
         .replace(Regex("\t"), "")
-        .replace("sun.misc.Unsafe.park", "jdk.internal.misc.Unsafe.park") // JDK8->JDK11
+        .replace("sun.misc.Unsafe.", "jdk.internal.misc.Unsafe.") // JDK8->JDK11
         .applyBackspace()
 
 public fun String.applyBackspace(): String {