Refactors OpenAPIService tests

[irvinesunday]

• Add test to validate we can retrieve all operations and paths.
• Removes test that validates root path is not present in PowerShell style. We already have a similar test for this:

  microsoft-graph-devx-api/OpenAPIService.Test/OpenAPIServiceShould.cs

  Lines 123 to 137 in cf8ab0c

  	[Fact]
  	public void ThrowsArgumentExceptionInApplyStyleWhenNoPathsAreReturned()
  	{
  	var predicate = _openApiService.CreatePredicate(operationIds: null,
  	tags: null,
  	url: "/",
  	source: _graphMockSource,
  	graphVersion: GraphVersion); // root path will be non-existent in a PowerShell styled doc.
  	var subsetOpenApiDocument = _openApiService.CreateFilteredDocument(_graphMockSource, Title, GraphVersion, predicate);
  	// Act & Assert
  	var message = Assert.Throws<ArgumentException>(() => _openApiService.ApplyStyle(OpenApiStyle.PowerShell, subsetOpenApiDocument)).Message;
  	Assert.Equal("No paths found for the supplied parameters.", message);
  	}

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