lib/string_map: fix return type of string_cmp

I can't figure out how checking the sign of a bool ever worked. The
following program demonstrates the problem (i.e. for me it prints 1).

 #include <stdio.h>
 #include <stdbool.h>
 int main(int argc, char **argv) {
    bool x;
    x = -1;
    printf("x = %d\n", x);
 }

This seems to be mandated by the C99 standard 6.3.1.2.

lib/string-map.c

@@ -106,7 +106,7 @@ _notmuch_string_map_sort (notmuch_string_map_t *map)
     map->sorted = true;
 }
 
-static bool
+static int
 string_cmp (const char *a, const char *b, bool exact)
 {
     if (exact)

test/T610-message-property.sh

@@ -101,7 +101,6 @@ EOF
 test_expect_equal_file EXPECTED OUTPUT
 
 test_begin_subtest "testing string map binary search (via message properties)"
-test_subtest_known_broken
 cat c_head - c_tail <<'EOF' | test_C ${MAIL_DIR}
 {
    char *keys[] = {"a", "b", "c", "d", "e", NULL};