Use runtime mode for testing

[NullVoxPopuli]

Copied from:

• https://github.com/NullVoxPopuli/ember.nvp/blob/909e80ee5a6ba0775f58f5da1624f83e55f2c18e/src/bases/minimal-app/files/babel.config.js#L4
• https://github.com/NullVoxPopuli/ember.nvp/blob/909e80ee5a6ba0775f58f5da1624f83e55f2c18e/src/layers/qunit/index.js#L41

To better handle this error:

error during build:
[babel] tests/test-helper.ts: setTesting(true) cannot change the testing state in compile-time mode. The current global config has isTesting=false. setTesting() calls are compiled away at build time, so they cannot change the testing state. If you need to change the testing state at runtime, use runtime mode instead.
   9 |
  10 | export function start() {
> 11 |   setTesting(true);
     |   ^^^^^^^^^^^^^^^^
  12 |   setApplication(Application.create(config.APP));
  13 |   configureNrg();
  14 |

so, when preparing to run tests in the CLI, we need to tell babel we want either:

• isTetsing to be true,
• or: enable runtime mode so it can be changed