Unify test strategy

[ajalab]

We recently added build_text to generate a text for tests.

fm-index/src/suffix_array/sais.rs

Lines 530 to 552 in a88db3b

	/// Build a text for tests using a generator function `gen`.
	fn build_text<T: Zero, F: FnMut() -> T>(mut gen: F, len: usize) -> Vec<T> {
	let mut text = (0..(len - 1)).map(|_| gen()).collect::<Vec<_>>();
	// Truncate the trailing zeros, since SA-IS does not support trailing zero suffix longer than 1.
	if let Some(pos) = text.iter().rposition(|x| !x.is_zero()) {
	text.truncate(pos + 1);
	} else {
	text.clear();
	}
	// Add non-zero elements until the text length reaches len - 1.
	while text.len() < len - 1 {
	let c = gen();
	if !c.is_zero() {
	text.push(c);
	}
	}
	// Add the last zero as a sentinel for SA-IS.
	text.push(T::zero());
	text
	}

This is specific to sais module, and in other places we adopt different strategies (use small texts or random texts) to generate test texts.

Let's unify these testing strategies.