MDEV-14309 MTR tests require perl-Env which is not always in the default installation

* don't use Env module in tests, use $ENV{xxx} instead
* collateral changes:
** $file in the error message was unset
** $file in the other error message was unset too :)
** source file arguments are conventionally upper-cased
** abort the test (die) on error, don't just echo/exit

mysql-test/include/truncate_file.inc

 # truncate a giving file, all contents of the file are be cleared
 
-if (!$file)
+if (!$TRUNCATE_FILE)
 {
-  --echo Please assign a file name to $file!!
-  exit;
+  die TRUNCATE_FILE is not set;
 }
 
-let TRUNCATE_FILE= $file;
-
 perl;
-use Env;
-Env::import('TRUNCATE_FILE');
-open FILE, '>', $TRUNCATE_FILE || die "Can not open file $file";
+open FILE, '>', $ENV{TRUNCATE_FILE} or die "open(>$ENV{TRUNCATE_FILE}): $!";
 close FILE;
 EOF

mysql-test/suite/rpl/t/rpl_manual_change_index_file.test

@@ -25,7 +25,7 @@ sync_slave_with_master;
 connection master;
 # Delete './master-bin.000001' from index file.
 let $MYSQLD_DATADIR= `SELECT @@DATADIR`;
-let $file= $MYSQLD_DATADIR/master-bin.index;
+let TRUNCATE_FILE= $MYSQLD_DATADIR/master-bin.index;
 source include/truncate_file.inc;
 
 if (`SELECT CONVERT(@@VERSION_COMPILE_OS USING latin1) NOT IN ('Win32', 'Win64', 'Windows')`)