Update testing libraries and migrate FluentAssertions syntax

[lahma]

With master and NET SDK 9 the restore process fails due to insecure dependencies in FluentAssertions and FakeItEasy dependency paths:

\YamlDotNet.Samples\YamlDotNet.Samples.csproj : error NU1903: Warning As Error: Package 'System.Net.Http' 4.3.0 has a known high severity vulnerability, https://github.com/advisories/GHSA-7jgj-8wvc-jh57
\YamlDotNet.Test\YamlDotNet.Test.csproj : error NU1903: Warning As Error: Package 'System.Net.Http' 4.3.0 has a known high severity vulnerability, https://github.com/advisories/GHSA-7jgj-8wvc-jh57
\YamlDotNet.Samples\YamlDotNet.Samples.csproj : error NU1903: Warning As Error: Package 'System.Text.RegularExpressions' 4.3.0 has a known high severity vulnerability, https://github.com/advisories/GHSA-cmhx-cq75-c4mj
\YamlDotNet.Test\YamlDotNet.Test.csproj : error NU1903: Warning As Error: Package 'System.Text.RegularExpressions' 4.3.0 has a known high severity vulnerability, https://github.com/advisories/GHSA-cmhx-cq75-c4mj

Upgraded to latest versions and fixed syntax.