Use Assert.Fail(message) as appropriate

Visual Studio pointed out that this coding pattern is preferred to
`Assert.True(false, message)`.

Signed-off-by: Johannes Schindelin <[email protected]>

src/shared/Core.Tests/HostProviderTests.cs

@@ -30,7 +30,7 @@ public async Task HostProvider_GetCredentialAsync_CredentialExists_ReturnsExisti
                 IsSupportedFunc = _ => true,
                 GenerateCredentialFunc = _ =>
                 {
-                    Assert.True(false, "Should never be called");
+                    Assert.Fail("Should never be called");
                     return null;
                 },
             };