How to create an irule for below mentioned requirement

You could do something like this:

when HTTP_REQUEST {
    switch [string tolower [HTTP::host]] {
        "mydomain.testdomain.com" {
            HTTP::respond "HTTP/1.1 301 Moved Permanently\r\nLocation:http://testdomain.com/mydomain/\r\nConnection: close\r\nContent-Length: 0\r\n\r\n"
        }
        "abc.testdomain.com" {
            HTTP::respond "HTTP/1.1 301 Moved Permanently\r\nLocation:http://testdomain.com/abc/\r\nConnection: close\r\nContent-Length: 0\r\n\r\n"
        }
        "def.testdomain.com" {
            HTTP::respond "HTTP/1.1 301 Moved Permanently\r\nLocation:http://testdomain.com/def/\r\nConnection: close\r\nContent-Length: 0\r\n\r\n"
        }
        "testing.testdomain.com" {
            HTTP::respond "HTTP/1.1 301 Moved Permanently\r\nLocation:http://testdomain.com/testing/\r\nConnection: close\r\nContent-Length: 0\r\n\r\n"
        }
    }
}

or like this depending on how dynamic you'd like your iRule to be.

when HTTP_REQUEST {
    if { not ([HTTP::host] equals "testdomain.com") } {
        set myDomain [getfield [HTTP::host] "." 1]
        HTTP::respond "HTTP/1.1 301 Moved Permanently\r\nLocation:http://testdomain.com/$myDomain/\r\nConnection: close\r\nContent-Length: 0\r\n\r\n"
    }
}