add fake-juniper-server.py and tests/juniper-auth

Flask-based tests of Juniper authentication forms handling. Currently tested cases are:

Standard (not SSO) login:

• frmLogin with username/password
• frmLogin with username/password/authgroup
• frmLogin with username/password/token-as-2nd-password
• frmLogin with username/password → frmTotpToken
• frmLogin with username/password → frmDefender → frmConfirmation
• frmLogin with username/password → frmNextToken
• frmLogin with username/password → frmSelectRoles → frmConfirmation

SSO login (thanks entirely to @jkuebart's contributions):

• loginForm with username/password → TOTP-generated VerificationCode → fake TNCC

Unlike fake-{f5,fortinet}-server.py (added in !169 (closed)), the fake-juniper-server.py here doesn't try to actually send mock IP/DNS configuration information to the client. It only handles the authentication phase, because that's the only part that actually happens over HTTPS.

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In addition to adding the test content, the base code's handling of several of these authentication options is improved, so that the tests fully pass:

1. Allow --authgroup to fill either the realm form field or the role ("pseudo-form-field") for Juniper.
2. Fix handling of token-field detection for Juniper SSO loginForm.
3. Make CLI autofill username in any field matching /^user/ case-insensitively (thus include the UserName field of Juniper SSO).