Tests: Use currency.NewBTCUSD and NewBTCUSDT (#1895)

* Tests: Use currency.NewUSD and NewUSDT

Simple refactor to use the provided shortcut methods

* Github: Add CI check to ensure NewPair not used

Add a step to ensure NewPair(BTC, USD*) isn't used

.github/workflows/misc.yml

@@ -0,0 +1,13 @@
+name: misc
+on: [push, pull_request]
+jobs:
+  lint:
+    name: miscellaneous checks
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - name: Check for currency.NewPair(BTC, USD) used instead of currency.NewBTCUSD
+      run: |
+        grep -r -n -E "currency.NewPair\(currency.BTC, currency.USDT?\)" * || exit 0
+        echo "::error::Replace currency.NewPair(BTC, USD*) with currency.NewBTCUSD*()"
+        exit 1