[DomCrawler][FrameworkBundle] Add `assertSelectorCount`

[fabpot]

Q	A
Feature PR	symfony/symfony#48424
PR author(s)	@curlycarla2004
Merged in	6.3

[OskarStark]

Are you open to tackle this one too @curlycarla2004?

[javiereguiluz]

Closing as fixed in #17640.