test-tables: allow sparse tables and check mapping for -1

Jan: test-tables fails on my system. The one it's failing on is:
    syscall: 222 → (null) → -1
... and indeed, our own tables should not have holes, but syscall
tables certainly might.

src/shared/test-tables.h

@@ -26,10 +26,11 @@ typedef int (*reverse_t)(const char*);
 static inline void _test_table(const char *name,
                                lookup_t lookup,
                                reverse_t reverse,
-                               int size) {
+                               int size,
+                               bool sparse) {
         int i;
 
-        for (i = 0; i < size + 1; i++) {
+        for (i = -1; i < size + 1; i++) {
                 const char* val = lookup(i);
                 int rev;
 
@@ -39,10 +40,12 @@ static inline void _test_table(const char *name,
                         rev = reverse("--no-such--value----");
 
                 printf("%s: %d → %s → %d\n", name, i, val, rev);
-                if (i < size ? val == NULL || rev != i : val != NULL || rev != -1)
+                if (i >= 0 && i < size ?
+                    sparse ? rev != i && rev != -1 : val == NULL || rev != i :
+                    val != NULL || rev != -1)
                         exit(EXIT_FAILURE);
         }
 }
 
 #define test_table(lower, upper) \
-        _test_table(STRINGIFY(lower), lower##_to_string, lower##_from_string, _##upper##_MAX)
+        _test_table(STRINGIFY(lower), lower##_to_string, lower##_from_string, _##upper##_MAX, false)

src/test/test-tables.c

@@ -99,7 +99,7 @@ int main(int argc, char **argv) {
         test_table(unit_load_state, UNIT_LOAD_STATE);
         test_table(unit_type, UNIT_TYPE);
 
-        _test_table("syscall", syscall_to_name, syscall_from_name, syscall_max());
+        _test_table("syscall", syscall_to_name, syscall_from_name, syscall_max(), true);
 
         return EXIT_SUCCESS;
 }