Rename formats-util.h to format-util.h

We don't have plural in the name of any other -util files and this
inconsistency trips me up every time I try to type this file name
from memory. "formats-util" is even hard to pronounce.

src/test/test-cgroup-util.c

@@ -21,7 +21,7 @@
 #include "cgroup-util.h"
 #include "dirent-util.h"
 #include "fd-util.h"
-#include "formats-util.h"
+#include "format-util.h"
 #include "parse-util.h"
 #include "process-util.h"
 #include "stat-util.h"

src/test/test-log.c

@@ -20,7 +20,7 @@
 #include <stddef.h>
 #include <unistd.h>
 
-#include "formats-util.h"
+#include "format-util.h"
 #include "log.h"
 #include "util.h"
 

src/test/test-tmpfiles.c

@@ -25,7 +25,7 @@
 #include "alloc-util.h"
 #include "fd-util.h"
 #include "fileio.h"
-#include "formats-util.h"
+#include "format-util.h"
 #include "fs-util.h"
 #include "log.h"
 #include "string-util.h"