avoid warning during lua unit test

[ElvinEfendi]

What this PR does / why we need it:
This is to avoid warning messages during Lua test runs such as:

2019/07/11 22:22:31 [warn] 118#118: *2 [lua] _G write guard:12: __newindex(): writing a global lua variable ('_TEST') which may lead to race conditions between concurrent requests, so prefer the use of 'local' variables
stack traceback:
	./rootfs/etc/nginx/lua/test/balancer_test.lua:1: in main chunk
	[C]: in function 'xpcall'
	/usr/local/openresty/luajit/share/lua/5.1/busted/core.lua:178: in function 'safe'
	/usr/local/openresty/luajit/share/lua/5.1/busted/block.lua:146: in function 'execute'
	/usr/local/openresty/luajit/share/lua/5.1/busted/init.lua:7: in function 'executor'
	/usr/local/openresty/luajit/share/lua/5.1/busted/core.lua:312: in function </usr/local/openresty/luajit/share/lua/5.1/busted/core.lua:312>
	[C]: in function 'xpcall'
	/usr/local/openresty/luajit/share/lua/5.1/busted/core.lua:178: in function 'safe'
	/usr/local/openresty/luajit/share/lua/5.1/busted/core.lua:312: in function 'execute'
	.../local/openresty/luajit/share/lua/5.1/busted/execute.lua:58: in function 'execute'
	/usr/local/openresty/luajit/share/lua/5.1/busted/runner.lua:197: in function </usr/local/openresty/luajit/share/lua/5.1/busted/runner.lua:11>
	./rootfs/etc/nginx/lua/test/run.lua:38: in function 'file_gen'
	init_worker_by_lua:58: in function <init_worker_by_lua:56>
	[C]: in function 'xpcall'
	init_worker_by_lua:65: in function <init_worker_by_lua:63>, context: ngx.timer

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

[aledbf]

/test pull-ingress-nginx-test

[aledbf]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, ElvinEfendi

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Needs approval from an approver in each of these files:

• OWNERS [ElvinEfendi,aledbf]

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[codecov-io]

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