Description

Add separator for Testnet accounts in Create account modal

Steps to reproduce

1. Enable testnets in Wallet network settings
2. Open Create account modal
3. There is just one list for all account types

Actual result

Expected result

Reproduces how often

Easily reproduced

Brave version (brave://version info)

All

Channel information

• release (stable)
• beta
• nightly

Reproducibility

• with Brave Shields disabled
• with Brave Rewards disabled
• in the latest version of Chrome

Miscellaneous information

cc: @Douglashdaniel @kleantzogu @jamesmudgett