u-string-list: move "test_split" into "u-string-list.c"

u-string-list: move "test_split" into "u-string-list.c"

We rely on "test-tool string-list" command to test the functionality of
the "string-list". However, as we have introduced clar test framework,
we'd better move the shell script into C program to improve speed and
readability.

Create a new file "u-string-list.c" under "t/unit-tests", then update
the Makefile and "meson.build" to build the file. And let's first move
"test_split" into unit test and gradually convert the shell script into
C program.

In order to create `string_list` easily by simply specifying strings in
the function call, create "t_vcreate_string_list_dup" function to do
this.

Then port the shell script tests to C program and remove unused
"test-tool" code and tests.

Signed-off-by: shejialuo <shejialuo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>