Coverage tool: add support for system properties getting set after JVM startup

[rliesenfeld]

The Maven Surefire plugin, when given a configuration containing system properties (using "systemProperties" or "systemPropertyVariables"), only sets them into the System.getProperties() object at some later time, not at JVM startup as would happen if "-D" arg-line parameters had been used.

This prevents jmockit from knowing about properties like "coverage-classes" and "coverage-excludes", when it's initialized with the "-javaagent" JVM parameter, since Surefire won't have set them yet. To avoid this problem, the properties need to be read at a later time, after Surefire has had the opportunity to set them.