Well the headlights, flashers, and everything else doesnt work!

Should I replace the battery even though it tested good?
How do i test if the grounding strap is doing its job?
Which is best for finding the fault light or multimeter?

If you've got a meter, there is a way to measure resistance in ohms. Select this capability and put one probe on the battery negative and one probe against the body on the inside of the hood. You should have continuity. If not, then your ground strap fell off the body somehow.