fix(entrypoint): tests fail on program crates

[yuwen01]

• cargo test usually creates a new "fn main" to be the entry point for running tests

• when we use !#[no_main] the test harness has unexpected behavior and just calls our main function

• you cant just not have a main symbol because rustc will complain

• compilation is currently broken in the template repo, and was fixed here 1c1f84d

• if were not targeting the zkvm, we return an empty main function that has return values that the harness will expect

• todo: ux? We should add a log somewhere if a user does try to make tests in the program crate

Originally from #1661

[github-actions]

SP1 Performance Test Results

Branch: yuwen/nathan-ci
Commit: d8f8059a
Author: yuwen01

program	cycles	execute (mHz)	core (kHZ)	compress (KHz)	time	success
fibonacci	11291	0.17	2.62	0.45	25s	✅
ssz-withdrawals	2757356	10.87	116.73	38.09	1m13s	✅
tendermint	12593597	5.38	237.25	106.84	2m0s	✅

[yuwen01]

#1659 CI is cooked in this PR, so i merged this one instead