enh: add stripe test keys to admin payment-gateway-form

[shreyanshdwivedi]

Fixes #3364

Changes proposed in this pull request:

• Adds stripe test keys to the payment-gateway-form

Test Keys

Live Keys

Checklist

• I have read the Contribution & Best practices Guide.
• My branch is up-to-date with the Upstream development branch.
• The acceptance, integration, unit tests and linter pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[shreyanshdwivedi]

@kushthedude @uds5501 @mrsaicharan1 @CosmicCoder96 please review

[kushthedude]

@shreyanshdwivedi Rather than computed property, Cant we use a boolean as done for Paypal Mode ?

[shreyanshdwivedi]

@kushthedude the server issue mentioned in the issue of this PR asked that we should use stripe test keys if the project is in debug environment. So I used computed property rather than a new variable stripeMode.