[SL.Core] Use Assert.Contains instead.

tests/SixLabors.Core.Tests/Helpers/DebugGuardTests.cs

@@ -59,7 +59,7 @@ namespace SixLabors.Helpers.Tests
             });
 
             Assert.Equal("myParamName", exception.ParamName);
-            Assert.True(exception.Message.Contains($"Value must be less than {max}."));
+            Assert.Contains($"Value must be less than {max}.", exception.Message);
         }
 
         [Theory]
@@ -79,7 +79,7 @@ namespace SixLabors.Helpers.Tests
             });
 
             Assert.Equal("myParamName", exception.ParamName);
-            Assert.True(exception.Message.Contains($"Value must be less than or equal to 1."));
+            Assert.Contains($"Value must be less than or equal to 1.", exception.Message);
         }
 
         [Fact]

tests/SixLabors.Core.Tests/Helpers/GuardTests.cs

@@ -25,7 +25,7 @@ namespace SixLabors.Helpers.Tests
             });
 
             Assert.Equal("myParamName", exception.ParamName);
-            Assert.True(exception.Message.Contains($"Value must be less than {max}."));
+            Assert.Contains($"Value must be less than {max}.", exception.Message);
         }
 
         [Theory]
@@ -45,7 +45,7 @@ namespace SixLabors.Helpers.Tests
             });
 
             Assert.Equal("myParamName", exception.ParamName);
-            Assert.True(exception.Message.Contains($"Value must be less than or equal to 1."));
+            Assert.Contains($"Value must be less than or equal to 1.", exception.Message);
         }
 
         [Fact]
@@ -85,7 +85,7 @@ namespace SixLabors.Helpers.Tests
             });
 
             Assert.Equal("myParamName", exception.ParamName);
-            Assert.True(exception.Message.Contains($"Value must be greater than or equal to 2."));
+            Assert.Contains($"Value must be greater than or equal to 2.", exception.Message);
         }
 
         [Theory]