Make tests works on systems with non english datetime format 

If you set the system Date and Time format to Italian( or any other culture with a different date format than MM/dd/yyyy) the type initializer for **NorthWindData** will throw an exception on:

```csharp
[...]
HireDate = ParseDate("8/14/1992 12:00:00 AM"), // the default it-IT date time format is dd/MM/yyyy
[..]
```
```
Exception details:

Test Name:	Any_simple
Test FullName:	Microsoft.Data.Entity.InMemory.FunctionalTests.NorthwindQueryTest.Any_simple
Test Source:	d:\MrJingle\Dev\Git\Forks\EntityFramework7\test\Shared\NorthwindQueryTestBase.cs : line 54
Test Outcome:	Failed
Test Duration:	0:00:00,001

Result Message:	
System.AggregateException : One or more errors occurred.
---- System.TypeInitializationException : The type initializer for 'Northwind.NorthwindData' threw an exception.
-------- System.FormatException : String was not recognized as a valid DateTime.
---- The following constructor arguments did not have matching fixture data: NorthwindQueryFixture fixture
Result StackTrace:	
----- Inner Stack Trace #1 (System.TypeInitializationException) -----
   at Microsoft.Data.Entity.InMemory.FunctionalTests.NorthwindQueryFixture..ctor() in d:\MrJingle\Dev\Git\Forks\EntityFramework7\test\Microsoft.Data.Entity.InMemory.FunctionalTests\NorthwindQueryTest.cs:line 39
----- Inner Stack Trace -----
   at System.DateTimeParse.Parse(String s, DateTimeFormatInfo dtfi, DateTimeStyles styles)
   at System.DateTime.Parse(String s)
   at Northwind.NorthwindData.ParseDate(String date) in d:\MrJingle\Dev\Git\Forks\EntityFramework7\test\Shared\Northwind.cs:line 303
   at Northwind.NorthwindData..cctor() in d:\MrJingle\Dev\Git\Forks\EntityFramework7\test\Shared\ **Northwind.cs:line 1590**
----- Inner Stack Trace #2 (Xunit.Sdk.TestClassException) -----
```

The fix is simply to change any occurrence of

```csharp
   DateTime.Parse("date string")
```
with

```csharp
   DateTime.Parse("date string", new CultureInfo("en-US"))
```