Resolve compiler warning from gen_password()

More recent g++ versions produce these warnings:
    test_PasswordRAMStore.cc: In member function ‘virtual void GParted::PasswordRAMStoreTest_TotalErasure_Test::TestBody()’:
    test_PasswordRAMStore.cc:61:32: warning: ‘                    ’ directive output truncated writing 20 bytes into a region of size 10 [-Wformat-truncation=]
      snprintf( buf, sizeof( buf ), "password%03u                    ", i );
                                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    test_PasswordRAMStore.cc:61:10: note: ‘snprintf’ output 32 bytes into a destination of size 21
      snprintf( buf, sizeof( buf ), "password%03u                    ", i );
      ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

snprintf() [1] truncates the printed string to the specified size so
didn't overflow the buffer.  However clear the warning by making the
formatted string always exactly 20 characters long, followed by the
terminating NUL character to exactly fill the buffer.

[1] print3(f) - Linux manual page
    https://man7.org/linux/man-pages/man3/snprintf.3.html
    "The functions snprintf() and vsnprintf() write at most size bytes
    (including the terminating null byte ('\0')) to str.
    "

tests/test_PasswordRAMStore.cc

@@ -58,7 +58,7 @@ static const char * gen_key( unsigned int i )
 static const char * gen_passwd( unsigned int i )
 {
 	static char buf[21];
-	snprintf( buf, sizeof( buf ), "password%03u                    ", i );
+	snprintf(buf, sizeof(buf), "password%03u         ", i%1000);
 	return buf;
 }