Refactored tests

[TestClass]
public class GravityTest
{
    private const decimal EarthFactor = 9.8M;
    private IPlanet Planet { get; set; }
    private Gravity Target { get; set; }
 
    [TestInitialize]
    public void BeforeEachTest()
    {
        Planet = Mock.Create<IPlanet>();
        Target = new Gravity(Planet);
    }
 
    [TestClass]
    public class GetVelocityAfter : GravityTest
    {
        [TestMethod]
        public void Returns_Zero_When_Passed_Zero()
        {
            AssertVelocityAfterNumberOfSeconds(numberOfSeconds: 0, expectedVelocity: 0);
        }
 
        [TestMethod]
        public void Returns_Nine_Point_Eight_After_1_Second_On_Earth()
        {
            Planet.Arrange(p => p.GetGravitationalFactor()).Returns(EarthFactor);
 
            AssertVelocityAfterNumberOfSeconds(numberOfSeconds: 1, expectedVelocity: 9.8);
        }
 
        [TestMethod]
        public void Returns_NinetyEight_After_10_Seconds_On_Earth()
        {
            Planet.Arrange(p => p.GetGravitationalFactor()).Returns(EarthFactor);
            
            AssertVelocityAfterNumberOfSeconds(numberOfSeconds: 10, expectedVelocity: 98);
            
        }
        
        private void AssertVelocityAfterNumberOfSeconds(int numberOfSeconds, decimal expectedVelocity)
        {
            var velocityAfterGivenSeconds = Target.GetVelocityAfter(numberOfSeconds);
 
            Assert.AreEqual<decimal>(expectedVelocity, velocityAfterGivenSeconds);
        
        }
    }
}

I think an assertion library like Should or Fluent Assertions can help to achieve the readability you are trying to get to without over-refactoring it:

using FluentAssertions;

[TestClass]
public class GravityTest
{
    private const decimal EarthFactor = 9.8M;
    private IPlanet Planet { get; set; }
    private Gravity Target { get; set; }

    [TestInitialize]
    public void BeforeEachTest()
    {
        Planet = Mock.Create<IPlanet>();
        Target = new Gravity(Planet);
    }

    [TestClass]
    public class GetVelocityAfter : GravityTest
    {
        [TestMethod]
        public void Returns_Zero_When_Passed_Zero()
        {
            var velocityAfterGivenSeconds = Target.GetVelocityAfter(0);

            velocityAfterGivenSeconds.Should().Be(0);
        }

        [TestMethod]
        public void Returns_Nine_Point_Eight_After_1_Second_On_Earth()
        {
            Planet.Arrange(p => p.GetGravitationalFactor()).Returns(EarthFactor);

            var velocityAfterGivenSeconds = Target.GetVelocityAfter(1);

            velocityAfterGivenSeconds.Should().Be(9.8);
        }
    }
}