Regress: Use --schema= option to change were the extension is installed

[Algunenano]

Currently the "—schema" option is used to check functions that might access objects or relations in the extension. We should extend it and use it also to install the extension into that schema.

[Algunenano]

The option was added in r17954

[strk]

In 17971:

[run_test] Use —schema as target for creating and finding postgis

Closes #4551

[Algunenano]

Related to this: We should change the files that we load for regress testing and, instead of reading the ones with @extschema@ stripped, we should replace @extschema@ with whatever schema is passed.

That way we can test pgplsql functions with something like:

BEGIN;
SET search_path = pg_catalog;
-- Query with the function
ROLLBACK;

in order to check that is independent on the search_path (e.g. everything is qualified properly)

[strk]

Let's go there incrementally, or file a ticket if you prefer