Disable MBT tests if the "mocks" feature is not enabled

[vitorenesduarte]

Closes: #642

Description

Before, running cargo test in the root folder would run the MBT tests successfully. However, running cargo test inside the modules/ folder doesn't work because the "mocks" is missing. This hints that running cargo test in the root folder is equivalent to running cargo test --all-features, but I couldn't find the corresponding documentation.

Ideally we would like to always enable the "mocks" feature when running tests, independently from where, but unfortunately this seems to not be supported: rust-lang/cargo#2911
This PR disables the MBT tests if the "mocks" feature is not enabled, which seemed to be the best workaround suggested in the previous issue: rust-lang/cargo#2911 (comment)

Now, running cargo test in the root folder still runs the MBT tests successfully, while running cargo test inside the modules/ folder simply ignores them. To also run the MBT tests inside the modules/ folder one has to:

cargo test --features mocks

This PR also adds more structure to the MBT tests, as suggested by @adizere in #601 (comment). This is a good suggestion since each file in the tests/ folder is compiled as it own crate. As a result, step.rs, modelator.rs and model_based.rs were all compiled individually, even though only the latter had a test in it. model_based.rs was renamed to mbt.rs just to minimize the diff in this PR.

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For contributor use:

• Updated the Unreleased section of CHANGELOG.md with the issue.
• If applicable: Unit tests written, added test to CI.
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Updated relevant documentation (docs/) and code comments.
• Re-reviewed Files changed in the Github PR explorer.

[adizere]

This makes sense. Thanks Vitor for the quick turnaround!

Could you also take care of the Changelog?

[codecov-io]

Codecov Report

Merging #643 (bb1062a) into master (b1b37f5) will increase coverage by 32.7%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #643      +/-   ##
=========================================
+ Coverage    13.6%   46.4%   +32.7%     
=========================================
  Files          69     152      +83     
  Lines        3752   10165    +6413     
  Branches     1374       0    -1374     
=========================================
+ Hits          513    4722    +4209     
- Misses       2618    5443    +2825     
+ Partials      621       0     -621     

Impacted Files	Coverage Δ
modules/src/address.rs	100.0% <ø> (ø)
...application/ics20_fungible_token_transfer/error.rs	0.0% <ø> (ø)
...pplication/ics20_fungible_token_transfer/events.rs	0.0% <ø> (ø)
...ion/ics20_fungible_token_transfer/msgs/transfer.rs	0.0% <ø> (ø)
modules/src/events.rs	0.0% <ø> (ø)
modules/src/handler.rs	100.0% <ø> (ø)
modules/src/ics02_client/client_def.rs	48.3% <ø> (ø)
modules/src/ics02_client/client_type.rs	79.1% <ø> (+31.5%)	⬆️
modules/src/ics02_client/context.rs	100.0% <ø> (ø)
modules/src/ics02_client/error.rs	100.0% <ø> (ø)
... and 260 more

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[vitorenesduarte]

Could you also take care of the Changelog?

Done in 361e046. There were two entries for BUG FIXES; I also fixed that.