[DomCrawler] Add assertFormValue() in WebTestCase

[mnapoli]

Q	A
Branch?	master for features
Bug fix?	no
New feature?	yes
Deprecations?	no
Tickets
License	MIT
Doc PR	symfony/symfony-docs#...

Add new assertions for checking form values, to use in functional tests.

Example:

    public function testForm()
    {
        $this->visit('/edit-form');

        self::assertNoFormValue('#form', 'activateMembership'); // checkboxes don't have values when unchecked
        self::assertFormValue('#form', 'trialPeriod', '15');

        $this->client->submitForm('Save', [
            'activateMembership' => 'on',
            'trialPeriod' => '7',
        ]);

        self::assertResponseIsSuccessful();
        self::assertFormValue('#form', 'activateMembership', 'on');
        self::assertFormValue('#form', 'trialPeriod', '7');
    }

I resorted to using the ->form() API because:

• checking the value of checkboxes was not possible today via existing assertions (I opened [DomCrawler] Add checkbox assertions for functional tests #38287 for that)
• checking the value of selects was impossible using CSS selectors (as far as I know), but possible with the ->form() helper

[fabpot]

Thank you @mnapoli.