enhancement request: improve plotIndiv() testing

[evaham1]

Is your feature request related to a problem? Please describe.

Running tests now produces lots of plots for 3d plot testing and also lots of warnings, primarily NAs introduced by coercion
Would like to keep decent coverage of plotIndiv() whilst cleaning up these plots and warnings during testing.

Also need to update this testing because it fails on aarch64 worker for Bioconductor building (https://bioconductor.org/checkResults/3.20/bioc-LATEST/mixOmics/). This is due to comparison of floating point integers as arithmetic operations with FP integers are not exact so need to check that the difference is within some range, with a tolerance e.g. https://stackoverflow.com/questions/61360074/compare-floats-in-r