t/helper: add an empty test-tool program

This will become an umbrella program that absorbs most [1] t/helper
programs in. By having a single executable binary we reduce disk usage
(libgit.a is replicated by every t/helper program) and shorten link
time a bit.

Running "make --jobs=1; du -sh t/helper" with ccache fully populated,
it takes 27 seconds and 277MB at the beginning of this series, 17
seconds and 42MB at the end.

[1] There are a couple programs that will not become part of
    test-tool: test-line-buffer and test-svn-fe have extra
    dependencies and test-fake-ssh's program name has to be a single
    word for some ssh tests.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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