E2E test suite should be parallelized

[pwschuurman]

The e2e test suite has 60 tests and runs on two separate machine types (gen-2 machines for PD types, and gen-4 machines for Hyperdisk types). These set up separate test contexts and run on separate VMs, so are totally execution independent.

The tests should be refactored to be parallelizeable, so the gen-2/gen-4 tests run concurrently. The test files can remain the same, but the test main (setup_e2e_test.go) should be pulled into separate packages, with machine type specific skips.

The test main will need to be moved to a separate packages (eg: gen-2 and gen-4 TestMain). This will also help with refactoring, as tests may need to be consolidated as older machine types are removed in the future.