tapchannel: allow greater rate range in unit conversion rapid test

We slightly bump the rapid test range for the asset rate, in order to
allow for 1 asset unit to evaluate to much less than 1msat. This exposes
some of the rounding errors causing errors in the rapid test (it fails
if you run it on this commit).

Author: GeorgeTsagk

tapchannel/aux_traffic_shaper_test.go

@@ -74,7 +74,7 @@ func TestUnitConversionToleranceRapid(t *testing.T) {
 			Draw(t, "invoiceAmtUnits")
 		numHTLCs := rapid.Uint64Range(1, 16).
 			Draw(t, "numHTLCs")
-		coefficient := rapid.Uint64Range(1, 20_000_000_000).
+		coefficient := rapid.Uint64Range(1, 300_000_000_000).
 			Draw(t, "coefficient")
 
 		rate := rfqmath.BigIntFixedPoint{