Add Bytecode test (ASM-based) to partest.

This commit introduces a new kind of test `Bytecode` that allows
one to inspect bytecode generated for given piece of Scala code.

The bytecode inspection is achieved by inspection of ASM trees.
See the included example for details.

NOTE: This commit does not introduce a new category of pratest tests.
Bytecode tests should be run in `jvm` category of partest tests.

Specific list of changes:
  * Add BytecodeTest that contains common utilities to partest
  * Add asm to classpath when compiling partest. That's not a
    new dependency as it's being already done for javac task
    we were running while compiling partest.
  * Add an example test that shows how to count null checks in
    given method.