[CN-Test-Gen] Add TESTING.md

Author: ZippeyKeys12

backend/cn/TESTING.md

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+# CN Testing
+
+CN has testing capabilities available via the `cn test` subcommand.
+
+## Overview
+
+Currently, CN supports only per-function tests, but additional types of testing may become available in the future.
+
+Running `cn test <filename.c>` generates C files with the testing infrastructure, the instrumented program under test, and a build script named `run_tests.sh`.
+This script compiles the C files and runs the tests.
+
+By default, running `cn test` will automatically run `run_tests.sh`, which produces a test executable `tests.out`.
+This can be disabled by using the `--no-run` flag.
+
+The output directory for these files can be set by using `--output-dir=<DIR>`.
+If the directory does not already exist, it is created.
+
+### Per-function tests
+
+When testing, there are currently two types of tests, constant tests and generator-based tests.
+For *each function with a body*, CN will create either a constant test or generator-based test.
+
+If a function takes no arguments, does not use accesses on global variables, and is correct, it should always return the same value and free any memory it allocates.
+In this case, a constant test is generated, which runs the function once and uses [Fulminate](FULMINATE_README.md) to check for post-condition violations.
+
+In all other cases, it creates generator-based tests, which are in the style of property-based testing.
+A "generator" is created, which randomly generates function arguments, values for globals accessed and heap states, all of which adhere to the given function's pre-condition.
+It calls the function with this input and uses [Fulminate](FULMINATE_README.md) similar to the constant tests.
+
+#### Understanding errors
+
+Currently, the tool does not print out counterexamples, but this is [planned](https://github.com/rems-project/cerberus/issues/841).
+Until then, `tests.out` can be run with the `--trap` flag in a debugger.
+Since seeds are printed each time the tool runs, `--seed <seed>` can be used to reproduce the test case.
+The debugger should automatically pause right before rerunning the failing test case.
+
+#### Writing custom tests
+
+There is currently no way to write custom property-based tests.
+However, once lemmas can be tested, a lemma describing the desired property could be written to test it.
+
+In terms of unit tests, one can simply define a function that performs the desired operations.
+This function will get detected by `cn test` and turned into a constant test.
+Any assertions that one would make about the result would have to be captured by the post-condition.
+In the future, existing infrastructure like `cn_assert` might be adapted for general use.