Improve YAML spec conformance by three tests

[am11]

• Relax the verison checks to support 1.1 to 1.3 range.
  • Fix up emitter to use version from DocumentStart object instead
    of constant.
• Add YAML 1.2 escape character '/'
  • This was the only difference in escaped sequence between 1.1 and
    1.2: https://yaml.org/spec/1.1/#id872840 and
    https://yaml.org/spec/1.2/spec.html#id2776092
• Improve grammar in SyntaxErrorException: find -> found to match
  with the counter part did not find (used in other cases).
• Improve formatting of failing test error message in SpecTests.
  • Sample failure:

    Test Name:	YamlDotNet.Test.Spec.SpecTests.ConformsWithYamlSpec(name: "4ABK", description: "Spec Example 7.17. Flow Mapping Separate Values", inputFile: "C:\\Users\\adeel\\Source\\Repos\\yamldotnet\\YamlD"..., expectedEventFile: "C:\\Users\\adeel\\Source\\Repos\\yamldotnet\\YamlD"..., error: False)
    Test FullName:	YamlDotNet.Test.Spec.SpecTests.ConformsWithYamlSpec (04719e7467f756f24522681db96dacf88dbb3cd7)
    Test Source:	C:\Users\adeel\Source\Repos\yamldotnet\YamlDotNet.Test\Spec\SpecTests.cs : line 51
    Test Outcome:	Failed
    Test Duration:	0:00:00.332
    
    Result StackTrace:	at YamlDotNet.Test.Spec.SpecTests.ConformsWithYamlSpec(String name, String description, String inputFile, String expectedEventFile, Boolean error) in C:\Users\adeel\Source\Repos\yamldotnet\YamlDotNet.Test\Spec\SpecTests.cs:line 65
    Result Message:
    Unexpected spec failure.
    Expected:
    +STR
    +DOC
    +MAP
    =VAL :unquoted
    =VAL "separate
    =VAL :http://foo.com
    =VAL :
    =VAL :omitted value
    =VAL :
    =VAL :
    =VAL :omitted key
    -MAP
    -DOC
    -STR
    
    Actual:
    [Writer Output]
    +STR
    +DOC
    +MAP
    =VAL :unquoted
    =VAL "separate
    
    [Exception]
    YamlDotNet.Core.SyntaxErrorException: (Line: 3, Col: 1, Idx: 27) - (Line: 3, Col: 5, Idx: 31): While scanning a plain scalar, found unexpected ':'.
    at YamlDotNet.Core.Scanner.ScanPlainScalar() in C:\Users\adeel\Source\Repos\yamldotnet\YamlDotNet\Core\Scanner.cs:line 1867
    at YamlDotNet.Core.Scanner.FetchPlainScalar() in C:\Users\adeel\Source\Repos\yamldotnet\YamlDotNet\Core\Scanner.cs:line 1817
    at YamlDotNet.Core.Scanner.FetchNextToken() in C:\Users\adeel\Source\Repos\yamldotnet\YamlDotNet\Core\Scanner.cs:line 480
    at YamlDotNet.Core.Scanner.FetchMoreTokens() in C:\Users\adeel\Source\Repos\yamldotnet\YamlDotNet\Core\Scanner.cs:line 213
    at YamlDotNet.Core.Scanner.MoveNextWithoutConsuming() in C:\Users\adeel\Source\Repos\yamldotnet\YamlDotNet\Core\Scanner.cs:line 127
    at YamlDotNet.Core.Parser.GetCurrentToken() in C:\Users\adeel\Source\Repos\yamldotnet\YamlDotNet\Core\Parser.cs:line 51
    at YamlDotNet.Core.Parser.ParseFlowMappingKey(Boolean isFirst) in C:\Users\adeel\Source\Repos\yamldotnet\YamlDotNet\Core\Parser.cs:line 896
    at YamlDotNet.Core.Parser.StateMachine() in C:\Users\adeel\Source\Repos\yamldotnet\YamlDotNet\Core\Parser.cs:line 187
    at YamlDotNet.Core.Parser.MoveNext() in C:\Users\adeel\Source\Repos\yamldotnet\YamlDotNet\Core\Parser.cs:line 115
    at YamlDotNet.Test.Spec.SpecTests.ConvertToLibYamlStyleAnnotatedEventStream(TextReader textReader, TextWriter textWriter) in C:\Users\adeel\Source\Repos\yamldotnet\YamlDotNet.Test\Spec\SpecTests.cs:line 96
    at YamlDotNet.Test.Spec.SpecTests.ConformsWithYamlSpec(String name, String description, String inputFile, String expectedEventFile, Boolean error) in C:\Users\adeel\Source\Repos\yamldotnet\YamlDotNet.Test\Spec\SpecTests.cs:line 60
    Expected: True
    Actual:   False
    

Fixes specs:

• UDM2 (Plain URL in flow mapping)
• 3UYS (Escaped slash in double quotes)
• 5MUD (Colon and adjacent value on next line)

Contributes to: #398

[aaubry]

Thanks!