mem.VirtualMemoryStat JSON fields capitalization and TestX_String tests

[Lomanic]

There are quite a few JSON fields all in lower case at mem/mem.go and following lines, whereas they only have their first capital letter in lower case in other packages and keep their initial capitalization. Shall we fix that in this package?

In the mean time, we should fix the TestX_String tests like this one to use reflection to dynamically get the fields instead of hard coding them inside the tests and to check their proper capitalization/name.