Fix various minor annoyances with running tests/test.sh interactively

• This fixes one issue where I noticed changes to tests/munit-tests/_YDBAIMTEST.m were not taking effect when I reran tests/test.sh interactively. This was because of the placement of the cp command that copied over the test M routines. So moved it to where it would cause recompilation as expected.

• Additionally I noticed an issue with tests/test.sh clean where if the $script_dir/downloads directory does not exist, one would get an error. This is now fixed by using rm -rf instead of rm -r.

• And finally noticed that the .gld file created by the test under the db directory would have references to a few regions through environment variables. This made it harder for one to look at all the regions through dse for example unless one also had those environment variables (ydb_dir and ydb_rel) set. This is now fixed by expanding those env vars at test run time so there is no reliance on env vars in the gld file.