Add regression test for return parameter on SP-bound SAML response (#2123)

# If applied, this commit will
extend the Bindings behat scenarios so the return parameter is
verified alongside RelayState for every SAML flow that posts a
response back to the SP.

# Why is this change needed?
Prior to this change, only RelayState was checked in the SAML POST
sent back to the SP. The return parameter had no equivalent coverage,
so a regression that reintroduces an empty return field (previously
fixed for the Pega Engine API issue) would go unnoticed.

# How does it address the issue?
This change adds an assertion to each of the six RelayState scenarios
in Bindings.feature confirming the process form has no return field
when it is posted to the SP, reusing the existing generic process-form
field step definitions.

# Provide links to any relevant tickets, articles or other resources
Closes #2056

Author: kayjoosten

src/OpenConext/EngineBlockFunctionalTestingBundle/Features/Bindings.feature

@@ -49,6 +49,7 @@ Feature:
       And I pass through the IdP
       And I give my consent
      Then the RelayState should be "test-relay-state-post-binding"
+      And The process form should not have the "return" field
       And I pass through EngineBlock
      Then the url should match "functional-testing/Dummy%20SP/acs"
 
@@ -60,6 +61,7 @@ Feature:
       And I pass through the IdP
       And I give my consent
      Then the RelayState should be "test-relay-state-redirect-binding"
+      And The process form should not have the "return" field
       And I pass through EngineBlock
      Then the url should match "functional-testing/Dummy%20SP/acs"
 
@@ -72,6 +74,7 @@ Feature:
       And I pass through the IdP
       And I give my consent
      Then the RelayState should be "test+state=with&special%20chars"
+      And The process form should not have the "return" field
       And I pass through EngineBlock
      Then the url should match "functional-testing/Dummy%20SP/acs"
 
@@ -85,6 +88,7 @@ Feature:
       And I pass through the IdP
       And I give my consent
      Then the RelayState should be "signed-request-relay-state"
+      And The process form should not have the "return" field
       And I pass through EngineBlock
      Then the url should match "functional-testing/Dummy%20SP/acs"
 
@@ -97,6 +101,7 @@ Feature:
       And I pass through the IdP
       And I give my consent
      Then no RelayState should be present
+      And The process form should not have the "return" field
       And I pass through EngineBlock
      Then the url should match "functional-testing/Dummy%20SP/acs"
 
@@ -108,6 +113,7 @@ Feature:
       And I pass through the IdP
       And I give my consent
      Then no RelayState should be present
+      And The process form should not have the "return" field
       And I pass through EngineBlock
      Then the url should match "functional-testing/Dummy%20SP/acs"