doc(test): suggest --no-build in conjunction with --ab

When running A/B tests, this part of the documentation explicitly builds
firecracker from two revisions. However, after that it will _still_
build Firecracker from HEAD _again_, even though this third compilation
of firecracker is not needed: The A/B test will only ever use the
precompiled binaries.

Fix this by passing --no-build in the example

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

tests/README.md

@@ -198,7 +198,7 @@ branch and the `HEAD` of your current branch, run
 ```sh
 tools/devtool -y build --rev main --release
 tools/devtool -y build --rev HEAD --release
-tools/devtool -y test --ab -- run build/main build/HEAD --test integration_tests/performance/test_boottime.py::test_boottime
+tools/devtool -y test --no-build --ab -- run build/main build/HEAD --test integration_tests/performance/test_boottime.py::test_boottime
 ```
 
 #### How to Write an A/B-Compatible Test and Common Pitfalls