javac reports "--add-opens has no effect at compile time"

[big-andy-coates]

When using the module plugin I'm seeing warnings being reported at when compiling test classes if the module-info.test file contains --add-opens lines.

e.g. from https://github.com/creek-service/creek-system-test, if I run:

>  gradle :model:testClasses

> Configure project :
Project :executor => 'creek.system.test.executor' Java module
Project :extension => 'creek.system.test.extension' Java module
Project :model => 'creek.system.test.model' Java module
Project :parser => 'creek.system.test.parser' Java module

> Task :model:compileTestJava
warning: [options] --add-opens has no effect at compile time
1 warning

BUILD SUCCESSFUL in 2s
6 actionable tasks: 6 executed

While the build passes, it would be nice to avoid these warnings by not passing --add-opens to javac.

Passing --add-opens to javac caused me to run into github/codeql-action#1040.