Fix to test fx APIs to retrieve correct var names from preview nodes

Preview values for constant nodes such as number nodes and string nodes are returned as 'null' as the preview variable names they were queried with did not match their symbol names. As a result of which there was a ```SymbolNotFoundException```. This is not an issue in normal execution as these nodes need not be previewed however this poses a problem in test frameworks, where they are previewed in tests. This is leading to a number of tests falsely failing.

This fix addresses this issue in the test framework locally.

@Benglin PTAL
Looks good, @aparajit-pratap, one minor point though...

Okay, here we expect a node to have at least one output, I'm not sure if we have a node without any output. But let's say we do in the future, with a node that processes its data and injects it into the 4th dimension for hyper space computation, the above logic will crash in our 3rd dimension.

I suggest that we added another ```Assert.IsGreaterThan(0)``` after that ```Assert.IsNotNull``` to avoid creating a space-time discontinuity which might result in the unit test case collapsing onto itself, in which case we will get a false true because the test case ends before it get started.

Other than that, LGTM.