tests: do not negate test_path_exists

As a way to assert the path 'foo' is missing, "! test_path_exists
foo" is a poor way to do so, as the helper is designed to complain
when 'foo' is missing, but the intention of the author who used
negated form was to make sure it does not exist.  This does not
help debugging the tests.

Use test_path_is_missing instead, which is a more appropriate helper.

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

t/t4067-diff-partial-clone.sh

@@ -151,7 +151,7 @@ test_expect_success 'diff does not fetch anything if inexact rename detection is
 
 	# Ensure no fetches.
 	GIT_TRACE_PACKET="$(pwd)/trace" git -C client diff --raw -M HEAD^ HEAD &&
-	! test_path_exists trace
+	test_path_is_missing trace
 '
 
 test_expect_success 'diff --break-rewrites fetches only if necessary, and batches blobs if it does' '
@@ -171,7 +171,7 @@ test_expect_success 'diff --break-rewrites fetches only if necessary, and batche
 
 	# Ensure no fetches.
 	GIT_TRACE_PACKET="$(pwd)/trace" git -C client diff --raw -M HEAD^ HEAD &&
-	! test_path_exists trace &&
+	test_path_is_missing trace &&
 
 	# But with --break-rewrites, ensure that there is exactly 1 negotiation
 	# by checking that there is only 1 "done" line sent. ("done" marks the

t/t4115-apply-symlink.sh

@@ -74,7 +74,7 @@ test_expect_success SYMLINKS 'symlink escape when creating new files' '
 	error: affected file ${SQ}renamed-symlink/create-me${SQ} is beyond a symbolic link
 	EOF
 	test_cmp expected_stderr stderr &&
-	! test_path_exists .git/create-me
+	test_path_is_missing .git/create-me
 '
 
 test_expect_success SYMLINKS 'symlink escape when modifying file' '