Reduce cyclomatic complexity of test_spectra() function to less than 15

[jmh579]

From usethis::linter():

[cyclocomp_linter] Functions should have cyclomatic complexity of less than 15, this has 32.

There are too many paths through the function, so it should be simplified.

• Cyclomatic complexity on Wikipedia
• cyclocomp R package

[jmh579]

Same problem with train_spectra(), but this one is even worse with a current cyclomatic complexity of 39!