tests: verify that particular expression did increased a counter

[quasilyte]

This will increase test suite effectiveness.
Probably should be only applied to negative tests.
Example:

func floatLit() {
	//= explicit-int/frac
	_ = 0.0
	//= explicit-int/frac
	_ = 0.123
	//= explicit-int/frac
	_ = 1.0
}