Added variants of 3 tests with 3 (MAX_SIGNERS) signers

[mariaKt]

This PR adds 3 test variants

• test_process_approve_multisig_3sig
• test_process_revoke_multisig_3sig
• test_process_withdraw_excess_lamports_multisig_multisig_3sig

which are the same as the

• test_process_approve_multisig
• test_process_revoke_multisig
• test_process_withdraw_excess_lamports_multisig_multisig
  except with 3 signers instead of 1. The reasoning is to choose a small number of proofs to be completed with 3 signers. These are not large and do not have multiple calls to validate_owner, so they should be manageable, and have different operations. On top of the test_process_initialize_multisig that already has 3, we will be doing 4/20.

[Stevengre]

We can merge it for speed. But I'm wondering if there any other better options.

[mariaKt]

EDIT: Removing test_process_approve_multisig_3sig.
These tests' proofs are very large, so I decided to only add two instead of three.