test(cst): cover the new eligibilty form

tests/pytest/eligibility/test_forms.py

@@ -1,4 +1,22 @@
-from benefits.eligibility.forms import MSTCourtesyCard, SBMTDMobilityPass
+from benefits.eligibility.forms import CSTAgencyCard, MSTCourtesyCard, SBMTDMobilityPass
+
+
+def test_CSTAgencyCard():
+    form = CSTAgencyCard(data={"sub": "12345", "name": "Gonzalez"})
+
+    assert form.is_valid()
+
+    sub_attrs = form.fields["sub"].widget.attrs
+    assert sub_attrs["pattern"] == r"\d{5}"
+    assert sub_attrs["inputmode"] == "numeric"
+    assert sub_attrs["maxlength"] == 5
+    assert sub_attrs["data-custom-validity"] == "Please enter a 5-digit number."
+
+    name_attrs = form.fields["name"].widget.attrs
+    assert name_attrs["maxlength"] == 255
+    assert name_attrs["data-custom-validity"] == "Please enter your last name."
+
+    assert form.use_custom_validity
 
 
 def test_MSTCourtesyCard():