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.

test/README

@@ -188,8 +188,8 @@ library for your script to use.
    is to summarize successes and failures in the test script and
    exit with an appropriate error code.
 
-There are also a number of mail-specific functions which are useful in
-writing tests:
+There are also a number of notmuch-specific auxiliary functions and
+variables which are useful in writing tests:
 
   generate_message
 
@@ -213,3 +213,15 @@ writing tests:
     will initialize the mail database to a known state of 50 sample
     messages, (culled from the early history of the notmuch mailing
     list).
+
+  notmuch_counter_reset
+  $notmuch_counter_command
+  notmuch_counter_value
+
+    These 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 prints the
+    current counter value.

test/test-lib.sh

@@ -919,6 +919,38 @@ test_emacs () {
 	emacsclient --socket-name="$EMACS_SERVER" --eval "(progn $@)"
 }
 
+# Creates a script that counts how much time it is executed and calls
+# notmuch.  $notmuch_counter_command is set to the path to the
+# generated script.  Use notmuch_counter_value() function to get the
+# current counter value.
+notmuch_counter_reset () {
+	notmuch_counter_command="$TMP_DIRECTORY/notmuch_counter"
+	if [ ! -x "$notmuch_counter_command" ]; then
+		notmuch_counter_state_path="$TMP_DIRECTORY/notmuch_counter.state"
+		cat >"$notmuch_counter_command" <<EOF || return
+#!/bin/sh
+
+read count < "$notmuch_counter_state_path"
+echo \$((count + 1)) > "$notmuch_counter_state_path"
+
+exec notmuch "\$@"
+EOF
+		chmod +x "$notmuch_counter_command" || return
+	fi
+
+	echo 0 > "$notmuch_counter_state_path"
+}
+
+# Returns the current notmuch counter value.
+notmuch_counter_value () {
+	if [ -r "$notmuch_counter_state_path" ]; then
+		read count < "$notmuch_counter_state_path"
+	else
+		count=0
+	fi
+	echo $count
+}
+
 test_reset_state_ () {
 	test -z "$test_init_done_" && test_init_