kunit: fix longest symbol length test

kunit: fix longest symbol length test

The kunit test that checks the longests symbol length [1], has triggered
warnings in some pilelines when symbol prefixes are used [2][3]. The test
will to depend on !PREFIX_SYMBOLS and !CFI_CLANG as sujested in [4] and
on !GCOV_KERNEL.

[1] https://lore.kernel.org/rust-for-linux/CABVgOSm=5Q0fM6neBhxSbOUHBgNzmwf2V22vsYC10YRBT=kN1g@mail.gmail.com/T/#t
[2] https://lore.kernel.org/all/20250328112156.2614513-1-arnd@kernel.org/T/#u
[3] https://lore.kernel.org/rust-for-linux/bbd03b37-c4d9-4a92-9be2-75aaf8c19815@infradead.org/T/#t
[4] https://lore.kernel.org/linux-kselftest/20250427200916.GA1661412@ax162/T/#t

Link: https://lore.kernel.org/r/20250706201855.232451-1-sergio.collado@gmail.com
Reviewed-by: Rae Moar <rmoar@google.com>
Signed-off-by: Sergio González Collado <sergio.collado@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>