Tmain: initialize "V" variable explicitly

[masatake]

"V" variable was introduced as a placeholder that is used when I want
to run the test case under valgrind; the valgrind command line is set
to "V".

It seems that "Ubuntu PPA build" environment defines V as an environment
variable with value "1". See #1761.

As the result, the test case doesn't work well on the environment.

This commit clears the value for "V" to allow the test case works
on the environment.

[hnakamur]

@masatake You are right. The failed build log contains the following:

User Environment
----------------

APT_CONFIG=/var/lib/sbuild/apt.conf
DEB_BUILD_OPTIONS=parallel=4
HOME=/sbuild-nonexistent
LANG=C.UTF-8
LC_ALL=C.UTF-8
LOGNAME=buildd
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games
SCHROOT_ALIAS_NAME=build-PACKAGEBUILD-14993131
SCHROOT_CHROOT_NAME=build-PACKAGEBUILD-14993131
SCHROOT_COMMAND=env
SCHROOT_GID=2501
SCHROOT_GROUP=buildd
SCHROOT_SESSION_ID=build-PACKAGEBUILD-14993131
SCHROOT_UID=2001
SCHROOT_USER=buildd
SHELL=/bin/sh
TERM=unknown
USER=buildd
V=1

Thanks for this commit! I imported this commit as a patch
hnakamur/universal-ctags-deb@8fe33d9
and build succeeded.
The build log

[masatake]

Thank you for verifying.