add e2e test for editing project length

frontend/e2e/cypress/integration/groupManagementPage.spec.js

@@ -74,6 +74,15 @@ describe('Group Management Page', () => {
       cy.get('.group-name').eq(0).should('contain', 'Ryhmä B')
     })
 
+    it('changes project length', () => {
+      cy.get('[data-cy=edit-project-length-short]').check()
+      cy.get('[data-cy=edit-group-save-button]').click()
+      cy.get('.notification').should('have.text', 'Group updated!')
+
+      cy.get('[data-cy=edit-group-button]').eq(0).click()
+      cy.get('[data-cy=edit-project-length-short]').should('be.checked')
+    })
+
     it('changes topic', () => {
       cy.get('.edit-group-form-topic__selector')
         .should('be.visible')

frontend/src/components/GroupManagementPage/SingleGroupEdit.jsx

@@ -169,9 +169,21 @@ class SingleGroupEdit extends React.Component {
           fullWidth={true}
         />
         <p>Change project length</p>
-        <RadioGroup name="length" value={this.state.projectLength} onChange={this.handleProjectLengthChange}>
-          <FormControlLabel value="short" control={<Radio />} label="short" />
-          <FormControlLabel value="long" control={<Radio />} label="long" />
+        <RadioGroup
+          name="length"
+          value={this.state.projectLength}
+          onChange={this.handleProjectLengthChange}
+        >
+          <FormControlLabel
+            value="short"
+            control={<Radio inputProps={{ 'data-cy': 'edit-project-length-short' }} />}
+            label="short"
+          />
+          <FormControlLabel
+            value="long"
+            control={<Radio inputProps={{ 'data-cy': 'edit-project-length-long' }} />}
+            label="long"
+          />
         </RadioGroup>
         <p>Add new students</p>
         <TextField