In order to fully replace testfor and simply return the number of entities existing, something like execute if entity @e[type=squid] or execute if entity @e[type=squid] run should work. However, it instead exhibits the following behavior:

if block: If the block is found, a syntax error is thrown. if the block is not found, nothing happens.
if entity: If the entity is found, a syntax error is thrown. If the entity is not found, the message is "No entity was found."
if score: If the score comparison succeeded, a syntax error is thrown. If the score comparison failed, nothing happens.
if blocks: If the regions match, a syntax error is thrown. If the regions do not match, nothing happens.

NOTE: Excluding run at the end of each of these commands after the relevant arguments results in a syntax error upon running.