All tests are located in /test and can be run using either truffle test with a local blockchain running, or truffle develop, and from the truffle console, test.
The goal for the tests is to validate that contracts' basic functionality works as expected.
Almost all tests were written in Javascript, though both the Marketplace and Stores contract have Solidity tests as well.
Files: TestMarketplace.sol, TestStores.sol
The Solidity tests are used to check that after deploying a contract, things are setup appropriately (ex: deployer is an admin, both contracts are properly linked, etc.).
Files marketplace.js, stores,js
The Javascript tests check that the contracts behave as expected. Most tests were written at the same time as the contract, before building the UI, to validate that things worked appropriately. As bugs were discovered by "manually testing" the contracts via the UI, tests were added to make sure they were fixed and that no regressions happened.
The tests are typically in the same order as the functions in the contracts. Elements that repeat across functions (ex: modifiers) are typically only tested once, and hence not against every single possible function they are applied to.