lib: add module support to string tests

Extract the string test code into its own source file, to allow
compiling it either to a loadable module, or built into the kernel.

Fixes: 03270c13c5ffaa6a ("lib/string.c: add testcases for memset16/32/64")
Link: http://lkml.kernel.org/r/1505397744-3387-1-git-send-email-geert@linux-m68k.org
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Matthew Wilcox <mawilcox@microsoft.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/lib/Kconfig b/lib/Kconfig
index a2b6745..368972f 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -584,7 +584,7 @@
 	tristate
 
 config STRING_SELFTEST
-	bool "Test string functions"
+	tristate "Test string functions"
 
 endmenu