tests: introduce lifecycle-stage fixture shadowing

Modules that need customisation between lifecycle stages - e.g. an extra
device or a file pushed into the guest - previously reimplemented the
whole spawn/basic_config/add_net_iface/start sequence locally. Document
same-name stage shadowing (including decoration via same-name
chaining) as a supported extension point in the fixture-system comment
block, and adopt it:

- test_rng.py: replace the local boot/restore constructors, the
  uvm_ctor parametrization, the rate_limiter fixture and the private
  uvm_any with two stage overrides: uvm_configured mirrors the conftest
  stage but spawns at INFO log level (preserving b755a67fb, "test: set
  log level to INFO for test_rng"), and uvm_booted configures the
  entropy device before boot.
- test_sysgenid.py, test_vmclock.py: decorate uvm_booted via
  same-name chaining to scp the guest test binary, deleting the local
  boot fixtures.
- test_fips.py: override uvm_configured to supply the command line
  parameters to the guest kernel, rather than maintaining an additional
  uvm_with_fips fixture.

Collected test variants are unchanged across all touched modules.

Signed-off-by: James Curtis <jxcurtis@amazon.co.uk>