…vscode-python#17747)

The behavior of interpreting "\n" inside quoted strings in env files
was neither documented nor tested, so this commit modifies the unit
tests to pin down that behavior.

I also added a test for the example I'm proposing to add to the docs
in PR microsoft/vscode-docs#4888.

I also extended the test for escaping "$" to show that it works even
when the "$" isn't part of something that matches
SUBST_REGEX (i.e. isn't the beginning of something looking like
"${...}").