Put kotlin-stdlib-jdk7/8 output in test classpath when running jdk6 tests

These tests ensure how compiled jdk6 binaries work in newer JDK7/8 environment,
and most likely the stdlib-jdk7/8 artifacts will be in that environment.

This reverts test classpath change (accidentally?) made in the commit 99f2cc3d.

libraries/stdlib/jdk7/build.gradle

@@ -101,7 +101,7 @@ task testJdk6Tests(type: Test) {
         testClassesDirs = project(':kotlin-stdlib').sourceSets.test.output
         classpath = files(
                 testClassesDirs,
-                configurations.testCompile
+                sourceSets.test.compileClasspath
         )
     }
 }

libraries/stdlib/jdk8/build.gradle

@@ -105,7 +105,7 @@ task testJdk6Tests(type: Test) { thisTask ->
         testClassesDirs = project(':kotlin-stdlib').sourceSets.test.output
         classpath = files(
                 testClassesDirs,
-                configurations.testCompile
+                sourceSets.test.compileClasspath
         )
     }
 }