use "GGally::ggcorr" in tests to avoid errors

[MLopez-Ibanez]

https://github.com/plotly/plotly.R/blob/555a443dcb1699977672c67e870d4e5a4dedf060/tests/testthat/test-plotly-subplot.R#L151

Error (test-plotly-subplot.R:150:3): ggplotly understands GGally
Error in `ggcorr(data, method = c("everything", "pearson"))`: could not find function "ggcorr"
Backtrace:
 1. plotly:::expect_doppelganger(...)
      at test-plotly-subplot.R:150:2
 2. plotly:::write_plotly_svg(p, path)
      at tests/testthat/helper-vdiffr.R:29:2
 3. plotly::plotly_build(p)
      at tests/testthat/helper-vdiffr.R:44:2

Just use "GGally::ggcorr"