test: make xmalloc.test tap-compatible

Part of #7618

NO_DOC=internal
NO_CHANGELOG=internal

(cherry picked from commit c573be80)

test/unit/xmalloc.c

+#define UNIT_TAP_COMPATIBLE 1
 #include "unit.h"
 #include "trivia/util.h"
 

test/unit/xmalloc.result

-	*** main ***
-1..5
-	*** test_xmalloc ***
-    1..1
-    ok 1 - p != NULL
-ok 1 - subtests
-	*** test_xmalloc: done ***
-	*** test_xcalloc ***
-    1..2
-    ok 1 - p != NULL
-    ok 2 - p is zeroed
-ok 2 - subtests
-	*** test_xcalloc: done ***
-	*** test_xrealloc ***
-    1..3
-    ok 1 - p != NULL on alloc
-    ok 2 - p != NULL on realloc
-    ok 3 - p is same after realloc
-ok 3 - subtests
-	*** test_xrealloc: done ***
-	*** test_xstrdup ***
-    1..3
-    ok 1 - s != NULL
-    ok 2 - copy != NULL
-    ok 3 - strcmp(s, copy) == 0
-ok 4 - subtests
-	*** test_xstrdup: done ***
-	*** test_xstrndup ***
-    1..6
-    ok 1 - s != NULL
-    ok 2 - copy != NULL
-    ok 3 - strlen(copy) == n
-    ok 4 - strncmp(s, copy, n) == 0
-    ok 5 - strncmp(s, copy, n + 1) > 0
-    ok 6 - strcmp(s, copy) > 0
-ok 5 - subtests
-	*** test_xstrndup: done ***
-	*** main: done ***