Add rspec definitions for riscv payloads

[bwatters-r7]

I'm getting a little crazy with the rspec inconsistencies. I'm seeing different results running things on Github and locally, and non-deterministic behavior all-around.
It does not help that the fix here is not considered a test failure (and is not listed as a failure), but is an error that will cause the tests to fail 😞
Locally, this change makes the branch pass (and also not error). I simply fail to understand why Github testing is complaining about payload cache size for windows payloads:

    0.83565 seconds ./spec/support/shared/examples/payload_cached_size_is_consistent.rb:113
  modules/payloads windows/shell/reverse_tcp_rc4 it should behave like payload cached size is consistent windows/shell/reverse_tcp_rc4 has a valid cached_size

Anyway, this PR fixes the error in rspec so even though everything passes, the tests will pass:

An error occurred in an `after(:context)` hook.
Failure/Error: expect(missing_ancestor_reference_name_set).to be_empty, "Some payloads are untested: #{missing_ancestor_reference_name_set.join(', ')}. Please update `modules/payloads_spec.rb` with the payload definitions"
  Some payloads are untested: stagers/linux/riscv32le/bind_tcp, stagers/linux/riscv32le/reverse_tcp, stagers/linux/riscv64le/bind_tcp, stagers/linux/riscv64le/reverse_tcp, stages/linux/riscv32le/shell, stages/linux/riscv64le/shell. Please update `modules/payloads_spec.rb` with the payload definitions
# ./spec/support/shared/contexts/untested_payloads.rb:62:in `block (2 levels) in <top (required)>'
...
Finished in 8 minutes 36 seconds (files took 9.15 seconds to load)
18794 examples, 0 failures, 33 pending, 1 error occurred outside of examples

Thanks for listening to me rant, and thank you for being patient.