STAT tool: Test more complex VFs, starting with Roboto Serif and Roboto Flex

[davelab6]

https://github.com/googlefonts/gftools/blob/main/Lib/gftools/tests/test_stat.py has currently 4 fixtures:

"""VF family consisting of a single font with two axes, wdth, wght"""

"""VF family consisting of two fonts, Roman, and Italic. Both have a
weight axis"""

"""VF family consisting of four fonts, Roman, Italic, Condensed Roman,
Condensed Italic. All only have a wght axis"""

"""VF family consisting of two fonts, Roman and Italic. Both have wdth and wght
axies."""

With Roboto Serif and Roboto Flex already past their "v1" releases, @rsheeter pointed out that it would be good to add more complex cases, like those two, plus expressive families like Recursive and Fraunces which have more non-MS-registered axes.