[JS IR] Build stdlib using compiler from repo

Previously JS IR versions of stdlib and kotlin-test were build
by default using compiler previously built on a buildserver.

It had some issues:
 - This required us to advance bootstrap every time we made any
   incompatible IR changes. This happens often since IR ABI is
   not quite stable yet.

 - We never tested the exact combination of compiler and stdlib we publish

   We tested:
    - new compiler with new stdlib build by new compiler (in box tests)
    - old compiler with new stdlib build by old compiler (in stdlib tests)

   We published:
    - new compiler with new stdlib build by old compiler

After this change JS IR compiler tests, builds and publishes
single configuration:

    new compiler with new stdlib build by new compiler

JS IR stdlib and kotlin-test are now built using JavaExec of CLI instead
of Gradle plugin to avoid troubles of loading a freshly built plugin.

This also allows to have a granular dependencies: we don't rebuild klib
if we changed a lowering in a compiler backend, but we do rebuild it if
we changed IR serialization algorithm.

libraries/kotlin.test/js/it/build.gradle

@@ -32,7 +32,11 @@ task populateNodeModules(type: Copy, dependsOn: compileKotlin2Js) {
 
     from {
         configurations.nodeModules.collect {
-            zipTree(it.absolutePath).matching { include '*.js' }
+             // WORKAROUND: Some JS IR jars were absent and caused this task to fail.
+             // They don't contain .js thus we can skip them.
+             if (it.exists()) {
+                 zipTree(it.absolutePath).matching { include '*.js' }
+             }
         }
     }