Fix test setup (#470)

Test project was applying Kotlin KMP plugin in subprojects, but not in the root 'build.gradle.kts' which is wrong from KMP point of view.

Author: Tapchicoma

integration-testing/examples/multi-module-test/build.gradle.kts

@@ -0,0 +1,3 @@
+plugins {
+    kotlin("multiplatform") version libs.versions.kotlinVersion.get() apply false
+}

integration-testing/examples/multi-module-test/consumer/build.gradle.kts

@@ -5,7 +5,7 @@
  */
 
 plugins {
-    kotlin("multiplatform") version libs.versions.kotlinVersion.get()
+    kotlin("multiplatform")
     application
 }
 

integration-testing/examples/multi-module-test/producer/build.gradle.kts

@@ -1,5 +1,5 @@
 plugins {
-    kotlin("multiplatform") version libs.versions.kotlinVersion.get()
+    kotlin("multiplatform")
     id("org.jetbrains.kotlinx.atomicfu") version libs.versions.atomicfuVersion.get()
 }