All tests for build scripts are under the build_test folder - I don't really think they should be edited unless there are any bugs/changes made to the build scripts.
These tests are in the wat_compiler folder.
The test cases for this repo can be found in the *.testcase.ts files. These files export an array of test cases, which are imported by the relevant *.test.ts files and run as parameterised tests.
To add tests, simply add a test case in the relevant *.testcase.ts file and append your test case to the exported array. Alternatively, you can create your own set of test cases and relevant parameterised test case.
For unit testing the lexer, parser, intermediate representations and binary writers (aka. all unit tests in the parse -> IR -> bytecode pipeline), they are tested in unit.test.ts. These unit tests use Jest's snapshot testing, mainly for ease of writing test cases. Run jest with the -u flag if there are any changes made (say, to the structure of IR classes) that warrants a change in these snapshots.
Integration tests are for testing the entire compilatio pipeline, in integration.test.ts. These tests compare the output of the TypeScript compiler with the official wabt compiler - any differences here should be considered a bug.