Avoid NoClassDefFoundError on JUnit Runner class

[rliesenfeld]

When the JRE lacks the native "attach" library and "-javaagent" is not used, JMockit initialization fails with the proper exception, but it also causes a NoClassDefFoundError for the org.junit.runner.Runner class (since it triggers initialization through the Attach API in the first place).

This secondary failure needs to be avoided, to avoid confusing the user.