update tests to pass go1.16

Author: quasilyte

_test/install/gitclone/expected6.txt

@@ -1 +1 @@
-/usr/local/go/src/encoding/base32/base32_test.go:99:4: exprUnparen: the parentheses around p.encoded[len(p.encoded)-1] == '=' are superfluous (rules.go:17)
+/usr/local/go/src/encoding/base32/base32_test.go:98:4: exprUnparen: the parentheses around p.encoded[len(p.encoded)-1] == '=' are superfluous (rules.go:17)

_test/install/gitclone/test.bash

@@ -27,8 +27,8 @@ diff -u actual.txt /root/expected4.txt
 ./go-ruleguard -e 'm.Match(`$f($*_, ($x), $*_)`)' /root/target.go &> actual.txt || true
 diff -u actual.txt /root/expected5.txt
 
-# TODO: make it work with `go mod vendor` properly.
-unset GO111MODULE
+# Run inside GOROOT.
+export GO111MODULE=off
 cd /usr/local/go
 go get -v -u github.com/quasilyte/go-ruleguard/dsl
 /root/go-ruleguard/go-ruleguard -rules /root/rules.go ./src/encoding/... &> actual.txt || true