Upgrade PHPUnit to 9.0

[morozov]

Q	A
Type	improvement
BC Break	yes

TODO:

• Backport VersionAwarePlatformDriver now extends Driver #3750 and PingableConnection and ServerInfoAwareConnection now extend Connection #3752 (see Remove support for doubling multiple interfaces sebastianbergmann/phpunit#3956).
• Rework the test that relies on assertContains() performing non-strict comparison internally (see Clean up assertContains() and assertNotContains() sebastianbergmann/phpunit#3426).
• Use expectExceptionMessageMatches() instead of expectExceptionMessageRegExp() ( see Remove expectExceptionMessageRegExp() sebastianbergmann/phpunit#3957).
• Use expectNotice() instead of expectException(Notice::class) (see Deprecate expectException(PHPUnit\Framework\Error\*) sebastianbergmann/phpunit#3776)