test: add functions to count how much times notmuch was called
The patch adds two auxiliary functions and a variable:
notmuch_counter_reset
$notmuch_counter_command
notmuch_counter_value
They allow to count how many times notmuch binary is called.
notmuch_counter_reset() function generates a script that counts how
many times it is called and resets the counter to zero. The function
sets $notmuch_counter_command variable to the path to the generated
script that should be called instead of notmuch to do the counting.
The notmuch_counter_value() function returns the current counter
value.