Test: read environment variables from external file

[weinand]

Test for #14523:

• Windows - @ramya-rao-a
• OS X - @bpasero
• Linux - @rebornix

Complexity: 2

The VS Code Node debugger now supports to load environment variables from a file and passes them to the node runtime.
To use this feature add an attribute envFile to your launch config and specify the absolute path to the file containing the environment variables:

   //...
   "envFile": "${workspaceRoot}/.env",
   "env": { "USER": "john doe" }
   //...

Any environment variable specified in env dictionary will override variables loaded from the file that have a matching name.

Example for env file:

USER=doe
PASSWORD=abc123

# a comment

# an empty value:
empty=

# new lines expanded in quoted strings:
lines="foo\nbar"

Verify that this feature works as specified.