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=[e-mail]/T/#t
[2] https://lore.kernel.org/all/[e-mail]/T/#u
[3] https://lore.kernel.org/rust-for-linux/[e-mail]/T/#t
[4] https://lore.kernel.org/linux-kselftest/20250427200916.GA1661412@ax162/T/#t

Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Rae Moar <[e-mail]>
Signed-off-by: Sergio González Collado <[e-mail]>
Acked-by: Randy Dunlap <[e-mail]>
Tested-by: Randy Dunlap <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

lib/Kconfig.debug

···
 config LONGEST_SYM_KUNIT_TEST
 	tristate "Test the longest symbol possible" if !KUNIT_ALL_TESTS
 	depends on KUNIT && KPROBES
+	depends on !PREFIX_SYMBOLS && !CFI_CLANG && !GCOV_KERNEL
 	default KUNIT_ALL_TESTS
 	help
 	  Tests the longest symbol possible

lib/tests/longest_symbol_kunit.c

···
  * Test the longest symbol length. Execute with:
  *  ./tools/testing/kunit/kunit.py run longest-symbol
  *  --arch=x86_64 --kconfig_add CONFIG_KPROBES=y --kconfig_add CONFIG_MODULES=y
- *  --kconfig_add CONFIG_RETPOLINE=n --kconfig_add CONFIG_CFI_CLANG=n
- *  --kconfig_add CONFIG_MITIGATION_RETPOLINE=n
+ *  --kconfig_add CONFIG_CPU_MITIGATIONS=n --kconfig_add CONFIG_GCOV_KERNEL=n
  */
 
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt