The Jedi don't know that the Sith Lord is a senator. Dooku only told Obi-Wan that senators were influenced by the Sith Lord:
Count Dooku: What if I told you that the Republic was now under the control of a Dark Lord of the Sith?
Obi-Wan: No, that's not possible. The Jedi would sense it.
Count Dooku: The dark side has clouded their vision. Hundreds of senators are now under the influence of a Sith Lord called Darth Sidious.
Obi-Wan: I don't believe you.
The Jedi can't be sure that Dooku is telling the truth about this (dark side users are deceitful), but this does spur the Jedi to keep a closer eye on the Senate:
Obi-Wan: Do you believe what Count Dooku said about Sidious controlling the Senate? It doesn't feel right.
Yoda: Becoming unreliable, Dooku has. Joined the dark side. Lies, deceit, creating mistrust are his ways now.
Mace Windu: Nevertheless, I feel we should keep a closer eye on the Senate.
Yoda: I agree.
Still, Dooku's words merely cause the Jedi to assume the Sith Lord is manipulating senators from behind the scenes (e.g. bribing them). Windu means that the Jedi need to determine which, if any, senators are indeed under the Sith Lord's influence -- not that they suspect the Sith Lord is a senator. The Jedi therefore have no reason to test the senators themselves for midi-chlorians.
The Jedi may not have the authority to subject senators to blood tests, either, especially on such flimsy evidence (hearsay from a dark side user, which doesn't even include the claim that the Sith Lord is a senator).