Add support for user friendly testing

[smiasojed]

Goals:

1. Enable contract authors to write unit tests in the same file as the contract.

// src/lib.rs
#![cfg_attr(not(feature = "std"), no_std, no_main)]

#[pvm_contract_sdk::contract]
pub mod flipper {
    pub struct Flipper { /* ... */ }
    impl Flipper {
        #[constructor] pub fn new(&mut self) { /* ... */ }
        #[method] pub fn flip(&mut self) { /* ... */ }
    }
}

#[cfg(all(test, feature = "std"))]
mod tests {
    use super::flipper::Flipper;
    use pvm_contract_sdk::MockHostBuilder;

    #[test]
    fn test_flip() {
        let mut f = Flipper::with_host(MockHostBuilder::new().build());
        f.flip();
        assert!(f.get());
    }
}

2. Allow to write e2e tests

[github-actions]

Benchmark Size Comparison (vs main)

Artifact	Baseline	Current	Delta	Status
fibonacci_builder-dsl.debug.polkavm	37345	37345	+0.00%	OK
fibonacci_builder-dsl.release.polkavm	638	638	+0.00%	OK
fibonacci_no-alloc.debug.polkavm	35967	35967	+0.00%	OK
fibonacci_no-alloc.release.polkavm	595	595	+0.00%	OK
fibonacci_with-alloc.debug.polkavm	46357	46357	+0.00%	OK
fibonacci_with-alloc.release.polkavm	600	600	+0.00%	OK
multi_builder-dsl.debug.polkavm	57629	57629	+0.00%	OK
multi_builder-dsl.release.polkavm	3024	3024	+0.00%	OK
multi_no-alloc.debug.polkavm	60352	60351	-0.00%	OK
multi_no-alloc.release.polkavm	2919	2919	+0.00%	OK
multi_with-alloc.debug.polkavm	71533	71532	-0.00%	OK
multi_with-alloc.release.polkavm	3076	3076	+0.00%	OK
mytoken_builder-dsl.debug.polkavm	55510	55510	+0.00%	OK
mytoken_builder-dsl.release.polkavm	3683	3683	+0.00%	OK
mytoken_no-alloc.debug.polkavm	59676	59678	+0.00%	OK
mytoken_no-alloc.release.polkavm	2922	2922	+0.00%	OK
mytoken_storage.debug.polkavm	63450	63458	+0.01%	OK
mytoken_storage.release.polkavm	3017	3017	+0.00%	OK
mytoken_with-alloc.debug.polkavm	73336	73343	+0.01%	OK
mytoken_with-alloc.release.polkavm	3943	3943	+0.00%	OK
----------------------------------------------------	------------	------------	----------	--------
Total	585572	585587	+0.00%

OK: All artifacts within 5% regression threshold

_{Run 26532801152 | ffa6b16 | 2026-05-27 19:22 UTC}