How to mock IConfiguration.GetValue

You can use an actual Configuration instance with in-memory data.

//Arrange
var inMemorySettings = new Dictionary<string, string> {
    {"TopLevelKey", "TopLevelValue"},
    {"SectionName:SomeKey", "SectionValue"},
    //...populate as needed for the test
};

IConfiguration configuration = new ConfigurationBuilder()
    .AddInMemoryCollection(inMemorySettings)
    .Build();


//...

Now it is a matter of using the configuration as desired to exercise the test

//...

string value = configuration.GetValue<string>("TopLevelKey");

string sectionValue = configuration.GetSection("SectionName").GetValue<string>("SomeKey");

//...

Reference: Memory Configuration Provider