[test] add more tests for formula w/ interactions, and for data w/ NA

[zhao-cy]

Add more tests:

• GAM formula with interaction terms:
  • ti(x, fx=TRUE) + ti(y, fx=FALSE) + ti(x, y, fx=TRUE) - however this is not recommended to perform effect size request (as not all fx=TRUE
  • s(x) + s(y) + ti(x,y)
• data with NA for ModelArray.lm() and ModelArray.gam():
  • use default na.action method to handle NA
    • also test for ModelArray.gam() with interaction term; did not test its effect size as NA observations should be dropped first
  • use na.action="na.omit" to handle NA (and results should be the same as default option)
  • use na.action="na.fail" to handle NA