`x-partial` warnings with GHC 9.8

[andreasabel]

src/Test/HUnit/Terminal.hs:41:41: warning: [GHC-63394] [-Wx-partial]
    In the use of ‘tail’
    (imported from Prelude, but defined in GHC.List):
    "This is a partial function, it throws an error on empty lists. Replace it with drop 1, or use pattern matching or Data.List.uncons instead. Consider refactoring to use Data.List.NonEmpty."
   |
41 |     | otherwise          = ta f (c:bs) (tail as) cs
   |                                         ^^^^

HUnit/src/Test/HUnit/Terminal.hs

Lines 40 to 41 in 95cbdaf

	| null as = ta f (c:bs) "" cs
	| otherwise = ta f (c:bs) (tail as) cs

This false alarm can be addressed by using drop 1 and merging the last two cases.

[sol]

PR welcome.