Allow customization of default scheduler name on JVM (#3465)

Fixes #3231

Author: IgnatBeresnev

kotlinx-coroutines-core/common/src/internal/SystemProps.common.kt

@@ -56,6 +56,17 @@ internal fun systemProp(
     return parsed
 }
 
+/**
+ * Gets the system property indicated by the specified [property name][propertyName],
+ * or returns [defaultValue] if there is no property with that key.
+ *
+ * **Note: this function should be used in JVM tests only, other platforms use the default value.**
+ */
+internal fun systemProp(
+    propertyName: String,
+    defaultValue: String
+): String = systemProp(propertyName) ?: defaultValue
+
 /**
  * Gets the system property indicated by the specified [property name][propertyName],
  * or returns `null` if there is no property with that key.

kotlinx-coroutines-core/jvm/src/scheduling/Tasks.kt

@@ -9,8 +9,13 @@ import kotlinx.coroutines.internal.*
 import java.util.concurrent.*
 
 
-// Internal debuggability name + thread name prefixes
-internal const val DEFAULT_SCHEDULER_NAME = "DefaultDispatcher"
+/**
+ * The name of the default scheduler. The names of the worker threads of [Dispatchers.Default] have it as their prefix.
+ */
+@JvmField
+internal val DEFAULT_SCHEDULER_NAME = systemProp(
+    "kotlinx.coroutines.scheduler.default.name", "DefaultDispatcher"
+)
 
 // 100us as default
 @JvmField