t3700: fix broken test under !SANITY

An "add --chmod=+x" test recently added by 610d55af0f ("add: modify
already added files when --chmod is given", 2016-09-14) used "xfoo3"
as a test file.  The paths xfoo[1-3] were used by earlier tests for
symbolic links but they were expected to have been removed by the
time the execution reached this new test.

The removal with "git reset --hard" however happened in a pair of
earlier tests, both of which are protected by POSIXPERM,SANITY
prerequisites.  Platforms and test environments that lacked these
would have seen xfoo3 as a leftover symbolic link that points at
somewhere else at this point of the sequence, and the chmod test
would have given a wrong result.

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

t/t3700-add.sh

@@ -350,6 +350,7 @@ test_expect_success POSIXPERM,SYMLINKS 'git add --chmod=+x with symlinks' '
 '
 
 test_expect_success 'git add --chmod=[+-]x changes index with already added file' '
+	rm -f foo3 xfoo3 &&
 	echo foo >foo3 &&
 	git add foo3 &&
 	git add --chmod=+x foo3 &&