test-drop-caches: use `has_dos_drive_prefix()`

This is a companion patch to 'mingw: handle `subst`-ed "DOS drives"':
use the DOS drive prefix handling that is already provided by
`compat/mingw.c` (and which just learned to handle non-alphabetical
"drive letters").

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

t/helper/test-drop-caches.c

@@ -6,18 +6,21 @@ static int cmd_sync(void)
 {
 	char Buffer[MAX_PATH];
 	DWORD dwRet;
-	char szVolumeAccessPath[] = "\\\\.\\X:";
+	char szVolumeAccessPath[] = "\\\\.\\XXXX:";
 	HANDLE hVolWrite;
-	int success = 0;
+	int success = 0, dos_drive_prefix;
 
 	dwRet = GetCurrentDirectory(MAX_PATH, Buffer);
 	if ((0 == dwRet) || (dwRet > MAX_PATH))
 		return error("Error getting current directory");
 
-	if ((Buffer[0] < 'A') || (Buffer[0] > 'Z'))
-		return error("Invalid drive letter '%c'", Buffer[0]);
+	dos_drive_prefix = has_dos_drive_prefix(Buffer);
+	if (!dos_drive_prefix)
+		return error("'%s': invalid drive letter", Buffer);
+
+	memcpy(szVolumeAccessPath, Buffer, dos_drive_prefix);
+	szVolumeAccessPath[dos_drive_prefix] = '\0';
 
-	szVolumeAccessPath[4] = Buffer[0];
 	hVolWrite = CreateFile(szVolumeAccessPath, GENERIC_READ | GENERIC_WRITE,
 		FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_EXISTING, 0, NULL);
 	if (INVALID_HANDLE_VALUE == hVolWrite)