enhancement request: expand test coverage for performance assessment functions

[evaham1]

Is your feature request related to a problem? Please describe.
Need to increase test coverage

Describe the solution you'd like
At least 50% coverage on all main mixOmics functions (i.e. the ones that build statistical tests like pca, pls, etc)
To see which functions need more coverage: https://app.codecov.io/gh/mixOmicsTeam/mixOmics/tree/master/R

• S3methods-plot.perf
• S3methods-plot.perf.pls
• S3methods-plot.tune
• auroc
• image.tune.rcc
• tune.pca
• tune.spls1
• tune.splsda
• withinVariation