Workaround in FloatMathTest.powers to avoid failures with js-ir runtime

Include kotlin-stdlib-js-ir in local core libs build/test and
enable failing build on test failures.

Relates to: KT-24975

libraries/stdlib/js-ir/build.gradle

@@ -193,9 +193,7 @@ task runMocha(type: NodeTask) {
     args += [testOutputFile, kotlinTestTestOutputFile]
 
     execOverrides {
-        // Tests are currently failing. Ignore the exit value for now
-        // TODO: replace with rootProject.ignoreTestFailures when tests are fixed
-        it.ignoreExitValue = true
+        it.ignoreExitValue = rootProject.ignoreTestFailures
         it.workingDir = buildDir
     }
 }