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.