Improvements and fixes in DateTimeFormatInfo

[josesimoes]

Description

• Remove duplicated strings.
• Tidy-up code.
• Add thorough unit tests for DateTime.ToString with predefined formats.

Motivation and Context

• Resolves DateTime.ToString() with custom formats is outputting wrong data Home#855.

How Has This Been Tested?

Screenshots

Types of changes

• Improvement (non-breaking change that improves a feature, code or algorithm)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Config and build (change in the configuration and build system, has no impact on code or features)
• Dependencies (update dependencies and changes associated, has no impact on code or features)
• Unit Tests (work on Unit Tests, has no impact on code or features)
• Documentation (changes or updates in the documentation, has no impact on code or features)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[nfbot]

Hi @josesimoes,

I'm nanoFramework bot.
Thank you for your contribution!

A human will be reviewing it shortly. 😉

[Ellerbach]

So I guess I should have run the code in culture info invariant, here are the correct output:

FullDateTimePattern:              dddd, dd MMMM yyyy HH:mm:ss
LongDatePattern:                  dddd, dd MMMM yyyy
LongTimePattern:                  HH:mm:ss
MonthDayPattern:                  MMMM dd
RFC1123Pattern:                   ddd, dd MMM yyyy HH':'mm':'ss 'GMT'
ShortDatePattern:                 MM/dd/yyyy
ShortTimePattern:                 HH:mm
SortableDateTimePattern:          yyyy'-'MM'-'dd'T'HH':'mm':'ss
UniversalSortableDateTimePattern: yyyy'-'MM'-'dd HH':'mm':'ss'Z'
YearMonthPattern:                 yyyy MMMM

[Ellerbach]

All good in invariant culture ;-)