Why is the Coulomb taken as the unit positve test charge, though $1 C$ charge is a high value? Why is the Coulomb taken as the unit positve charge, though $1 C$ charge is a high value?
 A: In days when people didn't know electrons even existed, they had to come up with a convenient way to define charge - something that would allow other people to reproduce the same number in another part of the world.
They came up with the Coulomb as "the charge that flows when 1 Ampere of current flows for 1 second".
Nice big round easy number. You can always add micro, nano, pico...
So - what make an Ampere easy to measure? Well two definitions - one the force exerted between two current carrying wires of a certain length and distance apart; another, the mass of silver deposited per unit time when electrolyzing a silver nitrate solution. These were experiments people could replicate elsewhere - and that is what ties it all together.
From wiki:

The "international ampere" was an early realization of the ampere, defined as the current that would deposit 0.001118000 grams of silver per second from a silver nitrate solution. Later, more accurate measurements revealed that this current is 0.99985 A.

A: A test charge is taken to represent one unit by convention.  Test charges are used to measure things like voltage or field.
If you use cgs units, you would put a franklin, and field (E) translates directly into dynes.  If you use SI units, a charge of 1 coulombs turns 'volts/metre' directly into 'newtons'.  
It's just a convention.  The coulomb/verber/franklin/biot-second, by being a test charge, is not supposed to intefere with what's being tested.
