Instead of copy-pasting full test driver for each value, make it to be
only one test with many values in table each tested by common driver.

TestZeroLengthData was checking output.BitLen() == 0 which is another
way to test for equality with big.Int(0) according to

	https://golang.org/pkg/math/big/#Int.BitLen

So join it too to the rest of decodeLong tests.

While at decodeLong topic, rename BenchmarkSpeed -> BenchmarkDecodeLong
as this benchmark checks speed of decodeLong only, nothing else.