[fuzz-utils] Add Path (2/6: Push,Pop)

This CL adds the Push and Pop method to Path class, which provides
the main means for the class to navigate the filesystem.

This CL also adds the test Fixture class, which is used to create
temporary files and directories under /tmp during unit testing. The
unit tests for Path have a derived PathFixture class for the files
needed in those tests; other unit tests may create their own fixtures.

SEC-103

Tests: Added more fuzz-utils unit tests for Path class

Change-Id: I520610f55dab80cb412ec4c1bb105907abd82014