Document how to run compat tests with SQLite

CONTRIBUTING.md

@@ -45,9 +45,20 @@ If the bytecode is the same, but query results are different, then the bug is so
 ## Compatibility tests
 
 The `testing/test.all` is a starting point for adding functional tests using a similar syntax to SQLite.
-When you run `make` the test cases are run with `cargo run`, which uses the latest build of the Limbo shell.
-If you run `testing/test.all` directly, it will run the tests with `sqlite3`.
 The purpose of these tests is to verify behavior matches with SQLite and Limbo.
+
+To run the test suite with Limbo, simply run:
+
+```
+make test
+```
+
+To run the test suite with SQLite, type:
+
+```
+SQLITE_EXEC=sqlite3 make test
+```
+
 When working on a new feature, please consider adding a test case for it.
 
 ## Deterministic simulation tests

Makefile

@@ -2,6 +2,9 @@ MINIMUM_RUST_VERSION := 1.73.0
 CURRENT_RUST_VERSION := $(shell rustc -V | sed -E 's/rustc ([0-9]+\.[0-9]+\.[0-9]+).*/\1/')
 RUSTUP := $(shell command -v rustup 2> /dev/null)
 
+# Executable used to execute the compatibility tests.
+SQLITE_EXEC ?= ./target/debug/limbo
+
 all: check-rust-version check-wasm-target limbo limbo-wasm
 .PHONY: all
 
@@ -39,5 +42,5 @@ limbo-wasm:
 .PHONY: limbo-wasm
 
 test: limbo
-	SQLITE_EXEC=./target/debug/limbo ./testing/all.test
-.PHONY: test
+	SQLITE_EXEC=$(SQLITE_EXEC) ./testing/all.test
+.PHONY: test