backend.native/tests: Add clean task to remove test output directory

backend.native/tests/build.gradle

@@ -8,21 +8,30 @@ dependencies {
     cli_bc project(path: ':backend.native', configuration: 'cli_bc')
 }
 
+def testOutputRoot = rootProject.file("testOutput").absolutePath
+
 allprojects {
     // Root directories for test output (logs, compiled files, statistics etc). Only single path must be in each set.
     sourceSets {
         // :backend.native:tests
         testOutputLocal {
-            output.dir(rootProject.file("testOutput/local"))
+           output.dir(rootProject.file("$testOutputRoot/local"))
         }
 
         // :backend.native:tests:external
         testOutputExternal {
-            output.dir(rootProject.file("testOutput/external"))
+            output.dir(rootProject.file("$testOutputRoot/external"))
         }
     }
 }
 
+task clean {
+    doLast {
+        delete(rootProject.file(testOutputRoot))
+    }
+}
+
+
 task regenerate_external_tests() {
     doLast {
         def externalTestsProject = childProjects["external"]