test: increase coverage build function

[AlexD10S]

Unit tests for the build parachain and build contracts processes.

Utilize mocks for the external crates that are used: duct::cmd for building parachain and contract_build::execute for building contracts.

[codecov]

Codecov Report

Attention: Patch coverage is 90.12346% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 57.81%. Comparing base (3cfff73) to head (c7f5804).
Report is 3 commits behind head on main.

@@            Coverage Diff             @@
##             main     #194      +/-   ##
==========================================
+ Coverage   55.97%   57.81%   +1.83%     
==========================================
  Files          33       36       +3     
  Lines        3398     3584     +186     
  Branches     3398     3584     +186     
==========================================
+ Hits         1902     2072     +170     
+ Misses       1213     1198      -15     
- Partials      283      314      +31     

Files	Coverage Δ
crates/pop-cli/src/main.rs	26.98% <ø> (ø)
crates/pop-cli/src/mock.rs	100.00% <100.00%> (ø)
crates/pop-contracts/src/mock.rs	100.00% <100.00%> (ø)
crates/pop-parachains/src/mock.rs	100.00% <100.00%> (ø)
crates/pop-contracts/src/build.rs	82.35% <90.00%> (+82.35%)	⬆️
crates/pop-parachains/src/build.rs	81.81% <80.00%> (+81.81%)	⬆️
crates/pop-cli/src/commands/build/contract.rs	70.37% <83.33%> (+70.37%)	⬆️
crates/pop-cli/src/commands/build/parachain.rs	75.75% <86.95%> (+75.75%)	⬆️

... and 3 files with indirect coverage changes

[Daanvdplas]

I idea is to perhaps place the parachain tests within this PR together, and the contracts tests. Now there is a split in the tests which can then perhaps be prevented. Not completely sure though