Make ClassLoaderValue compatible with PowerMock

[jglick]

Amending #370. jenkinsci/email-ext-plugin#311 etc. were filed because it turned out there were problems running plugin-compat-tester against plugins still using PowerMock (discouraged: jenkinsci/plugin-pom#442):

java.lang.ExceptionInInitializerError
	at java.base/java.lang.Class.forName0(Native Method)
	at java.base/java.lang.Class.forName(Class.java:315)
	at javassist.runtime.Desc.getClassObject(Desc.java:72)
	at javassist.runtime.Desc.getClassType(Desc.java:181)
	at javassist.runtime.Desc.getType(Desc.java:151)
	at javassist.runtime.Desc.getType(Desc.java:107)
	at io.jenkins.blueocean.service.embedded.BaseTest.<init>(BaseTest.java:46)
	at io.jenkins.blueocean.service.embedded.ContainerFilterTest.<init>(ContainerFilterTest.java:34)
	at …
Caused by: java.lang.IllegalArgumentException: org.kohsuke.stapler.ClassLoaderValue$X referenced from a method is not visible from class loader
	at java.base/java.lang.reflect.Proxy$ProxyBuilder.ensureVisible(Proxy.java:858)
	at java.base/java.lang.reflect.Proxy$ProxyBuilder.validateProxyInterfaces(Proxy.java:681)
	at java.base/java.lang.reflect.Proxy$ProxyBuilder.<init>(Proxy.java:627)
	at java.base/java.lang.reflect.Proxy$ProxyBuilder.<init>(Proxy.java:635)
	at java.base/java.lang.reflect.Proxy.lambda$getProxyConstructor$0(Proxy.java:415)
	at java.base/jdk.internal.loader.AbstractClassLoaderValue$Memoizer.get(AbstractClassLoaderValue.java:329)
	at java.base/jdk.internal.loader.AbstractClassLoaderValue.computeIfAbsent(AbstractClassLoaderValue.java:205)
	at java.base/java.lang.reflect.Proxy.getProxyConstructor(Proxy.java:413)
	at java.base/java.lang.reflect.Proxy.getProxyClass(Proxy.java:384)
	at org.kohsuke.stapler.ClassLoaderValue.get(ClassLoaderValue.java:55)
	at org.kohsuke.stapler.MetaClassLoader.get(MetaClassLoader.java:53)
	at org.kohsuke.stapler.MetaClassLoader.<init>(MetaClassLoader.java:46)
	at org.jvnet.hudson.test.JenkinsRule.<clinit>(JenkinsRule.java:2792)
	... 31 more

Debugging shows that cl was e.g. AppClassLoader whereas X.class was from org.powermock.core.classloader.javassist.JavassistMockClassLoader. Use the version loaded from cl.

[jglick]

BTW there is a ClassLoaderValue in Java 9+ which looks like it would work, but it is private.