Be extra lenient for non-Sun JDK perf tests

The slack factor for non-Sun JDK performance test in PackagingDataCalculatorTest
was not lenient enough for the small virtual nodes on Cloudbees Jenkins.
Some people were also reporting test failures here by a few percentage points
(on their slow development machines).

Bump slack factor from 10 to 15

(cherry picked from commit 841d22a)

logback-classic/src/test/java/ch/qos/logback/classic/spi/PackagingDataCalculatorTest.java

@@ -90,7 +90,7 @@ public void perfTest() {
     int slackFactor = 8;
     if (!SystemInfo.getJavaVendor().contains("Sun")) {
       // be more lenient with other JDKs
-      slackFactor = 10;
+      slackFactor = 15;
     }
     assertTrue("computing class packaging data (" + d1
         + ") should have been less than " + slackFactor