Redirection issues

The URLs you've shared - https://test.com && https://testapp.com

But your Irule is of URI. Refer WIKI.

Eg: https://[HTTP::host][HTTP::uri]"

So you should be capturing the host instead of URI.

when HTTP_REQUEST {
    switch [string tolower [HTTP::host]] {
        "test.com " {
            pool test_pool
        }
        "testapp.com" {
            pool testapp_pool
        }
    }
}

Your requirement is still not clear because of alignment. Not sure, but you may wanna be using

SNI feature