Create a Decimal ApproxEq function

[ValarDragon]

Summary

Similar to the DecEq function for testing,

cosmos-sdk/types/decimal.go

Lines 800 to 803 in d83a3bf

	// intended to be used with require/assert: require.True(DecEq(...))
	func DecEq(t *testing.T, exp, got Dec) (*testing.T, bool, string, string, string) {
	return t, exp.Equal(got), "expected:\t%v\ngot:\t\t%v", exp.String(), got.String()
	}

, we should also have a decimal ApproxEq function that checks (pseudocode):

DecApproxEq(t *testing.T, d1 sdk.Dec, d2 sdk.Dec, tolerance sdk.Dec) {
  absdiff := d1.Sub(d2).Abs()
  t.RequireLTE(absdiff, tolerance)
}

Basically checking that d1 is approximately equal to d2, within some tolerance bound.

Problem Definition

Simplifies lots of decimal testing code.

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