rocksdb_sys_vars should not be run if MyRocks is not compiled.

Copy the detection logic from 'rocksdb' test suite, add a note.

storage/rocksdb/mysql-test/rocksdb/suite.pm

 package My::Suite::Rocksdb;
 
+#
+# Note: ../rocksdb_sys_vars/suite.pm file has a similar
+#  function. If you modify this file, consider modifying that one, too.
+#
 @ISA = qw(My::Suite);
 use My::Find;
 use File::Basename;

storage/rocksdb/mysql-test/rocksdb_sys_vars/suite.pm

 package My::Suite::Rocksdb_sys_vars;
 
+#
+# Note: The below is copied from ../rocksdb/suite.pm
+#
 @ISA = qw(My::Suite);
+use My::Find;
+use File::Basename;
+use strict;
 
 sub is_default { not $::opt_embedded_server }
 
+my $sst_dump=
+::mtr_exe_maybe_exists(
+  "$::bindir/storage/rocksdb$::opt_vs_config/sst_dump",
+  "$::path_client_bindir/sst_dump");
+return "RocksDB is not compiled, no sst_dump" unless $sst_dump;
+$ENV{MARIAROCKS_SST_DUMP}="$sst_dump";
+
 bless { };