Define a function outside a class and call the function using action or filter hook

So I presumed your sub plugin loads after the main one, right?

Here i can see that new value is added to array but now i have to define test2 function inside my sub plugin

If you’re certain test2 successfully added to the array and that test2 was being called from MyClass::autoload_function(), then you can try using an early hook like init to define test2, like so:

<?php
/* Plugin Name: My Sub Plugin */

// Defines the test2() function.
function my_define_test2() {
    function test2(){
        echo 'hii- test2';
    }
}

// Now hook on init to ensure test2() is defined before your main plugin
// is loaded.
add_action( 'init', 'my_define_test2', 1 );

// And then add test2() to $my_array (that you defined in the main plugin).
add_filter( 'modify_array', 'modify_array_function' );
function modify_array_function( $my_array ) {
    array_push( $my_array, 'test2' );
    return $my_array; // remember, filters must always return something =)
}

If however MyClass is instantiated immediately when WordPress loads the plugin, i.e. something like so, then I don’t think it’s possible for you to define test2 before MyClass is instantiated. Unless maybe, if you use a Must Use plugin.

<?php
/* Plugin Name: My Main Plugin */

class MyClass { ... }

$var = new MyClass();