Split java 8 and 9 classpaths in TestClasspath

In preparation for adding the system modules jars to the classpath
for java 9, split the expected java 8 and java 9 classpaths into
separate fields.  Also replace the system: "bootclasspath" special
case with a bool that specifies that the module will always be
built using a java 8 style command line.

Test: TestClasspath
Change-Id: I661ec50d77a039c435c03bc8e6443cc5a57c206e