Automatically add --enable-preview to vmargs, when necessary

[fbricon]

Following on redhat-developer/vscode-java#671, when trying to test Java 12 code using preview features, you need to add the --enable-preview flag in the test configuration's vmargs, in settings.json:

    "java.test.config": [
        {
            "name": "java12",
            "workingDirectory": "${workspaceFolder}",
            "vmargs": [ "--enable-preview" ],
        },
    ],
    "java.test.defaultConfig": "java12"

This could be done automatically by the vscode-java-test extension. Also see microsoft/vscode-java-debug#553

[yllekz]

+1 to this. This has been driving me crazy. Programs previously functioning under JDK 8 suddenly give me all this hassle under JDK 12.

[legrassy]

+1

[stubifox]

+1 :)