style: neater leverage unit tests

[toteki]

Extracted from review on
#1654

maybe we can have a helper function:

type balances struct {
  balance, collateral, utokenSupply, exchangeRate, borrowed
}
func getBalances(ctx, addr) balances
And use it across test functions :

before := getBalances(...)
...
after := getBalances(...)
Will make the code easier to read (and much shorter)

let's create a helper function:

check := func(err, expectedDec, p) {...}
...
check(err, "0.5", p)

let's create a helper function ucoin which will do the "u/"+, and panic if denom is already a utoken,

please provide more meaningful name for [each test case]

[add comments for test cases that might be confusing in why they should fail/pass]

--- tcs := []testCase{
+++ tcs := []struct {
same for other test cases, makes it easier to read if we have type definition right next to the values.