./test: Add PASSES="mod_tidy" check

Added check that ensures that go.mod & go.sum files are up-to-date.
The check verifies whether 'go mod tidy' does not generate any mutations
in these files.
The check can be run on its own:
  PASSES="mod_tidy" ./test
Or as part of "fmt" pass:
  PASSES="fmt" ./test

Examplar outputs:

```
% PASSES="fmt" ./test

Running with TEST_CPUS: 1,2,4
Starting 'fmt' pass at Fri 11 Sep 2020 11:07:54 PM CEST
'shellcheck' started at Fri 11 Sep 2020 11:07:54 PM CEST
'shellcheck' completed at Fri 11 Sep 2020 11:07:54 PM CEST
'markdown_you' started at Fri 11 Sep 2020 11:07:54 PM CEST
'markdown_you' completed at Fri 11 Sep 2020 11:07:54 PM CEST
'goword' started at Fri 11 Sep 2020 11:07:54 PM CEST
'goword' completed at Fri 11 Sep 2020 11:07:54 PM CEST
'gofmt' started at Fri 11 Sep 2020 11:07:54 PM CEST
'gofmt' completed at Fri 11 Sep 2020 11:07:55 PM CEST
'govet' started at Fri 11 Sep 2020 11:07:55 PM CEST
'govet' completed at Fri 11 Sep 2020 11:07:57 PM CEST
'revive' started at Fri 11 Sep 2020 11:07:57 PM CEST
Skipping revive...
'revive' completed at Fri 11 Sep 2020 11:07:57 PM CEST
'license_header' started at Fri 11 Sep 2020 11:07:57 PM CEST
'license_header' completed at Fri 11 Sep 2020 11:07:58 PM CEST
'receiver_name' started at Fri 11 Sep 2020 11:07:58 PM CEST
'receiver_name' completed at Fri 11 Sep 2020 11:07:58 PM CEST
'commit_title' started at Fri 11 Sep 2020 11:07:58 PM CEST
'commit_title' completed at Fri 11 Sep 2020 11:07:58 PM CEST
'mod_tidy' started at Fri 11 Sep 2020 11:07:58 PM CEST
*** /tmp/fileiALKRA_go.mod	2020-09-11 23:07:58.838010716 +0200
--- ./go.mod	2020-09-11 23:07:58.974010922 +0200
***************
*** 29,39 ****
  	github.com/mattn/go-runewidth v0.0.2 // indirect
  	github.com/modern-go/reflect2 v1.0.1
  	github.com/olekukonko/tablewriter v0.0.0-20170122224234-a0225b3f23b5
  	github.com/prometheus/client_golang v1.0.0
  	github.com/prometheus/client_model v0.0.0-20190812154241-14fe0d1b01d4
- 	github.com/prometheus/common v0.4.1
  	github.com/sirupsen/logrus v1.4.2 // indirect
  	github.com/soheilhy/cmux v0.1.4
  	github.com/spf13/cobra v0.0.3
  	github.com/spf13/pflag v1.0.1
  	github.com/tmc/grpc-websocket-proxy v0.0.0-20170815181823-89b8d40f7ca8
--- 29,38 ----
./go.mod is not in sync with 'go mod tidy'
```

```
% PASSES="mod_tidy" ./test

Running with TEST_CPUS: 1,2,4
Starting 'mod_tidy' pass at Fri 11 Sep 2020 11:09:21 PM CEST
*** /tmp/file9gy4so_go.mod	2020-09-11 23:09:21.166133290 +0200
--- ./go.mod	2020-09-11 23:09:21.286133466 +0200
***************
*** 29,39 ****
  	github.com/mattn/go-runewidth v0.0.2 // indirect
  	github.com/modern-go/reflect2 v1.0.1
  	github.com/olekukonko/tablewriter v0.0.0-20170122224234-a0225b3f23b5
  	github.com/prometheus/client_golang v1.0.0
  	github.com/prometheus/client_model v0.0.0-20190812154241-14fe0d1b01d4
- 	github.com/prometheus/common v0.4.1
  	github.com/sirupsen/logrus v1.4.2 // indirect
  	github.com/soheilhy/cmux v0.1.4
  	github.com/spf13/cobra v0.0.3
  	github.com/spf13/pflag v1.0.1
  	github.com/tmc/grpc-websocket-proxy v0.0.0-20170815181823-89b8d40f7ca8
--- 29,38 ----
./go.mod is not in sync with 'go mod tidy'
```

test

@@ -627,6 +627,7 @@ function fmt_pass {
 			license_header \
 			receiver_name \
 			commit_title \
+			mod_tidy \
 			; do
 		echo "'$p' started at $(date)"
 		"${p}"_pass "$@"
@@ -684,6 +685,35 @@ function build_pass {
 	GO_BUILD_FLAGS="-v" tools_build
 }
 
+function mod_tidy_pass {
+	# Watch for upstream solution: https://github.com/golang/go/issues/27005
+	local tmpFileGoMod=$(tempfile -s "_go.mod")
+	local tmpFileGoSum=$(tempfile -s "_go.sum")
+  cp "./go.mod" "${tmpFileGoMod}"
+  cp "./go.sum" "${tmpFileGoSum}"
+  go mod tidy
+
+  set +e
+  diff -C 5 "${tmpFileGoMod}" "./go.mod"
+  local tmpFileGoModInSync="$?"
+  diff -C 5 "${tmpFileGoSum}" "./go.sum"
+  local tmpFileGoSumInSync="$?"
+  set -e
+
+  # Bring back initial state
+  mv "${tmpFileGoMod}" "./go.mod"
+  mv "${tmpFileGoSum}" "./go.sum"
+
+  if [ "${tmpFileGoModInSync}" -ne 0 ]; then
+		echo "./go.mod is not in sync with 'go mod tidy'"
+		exit 255
+	fi
+	if [ "${tmpFileGoSumInSync}" -ne 0 ]; then
+		echo "./go.sum is not in sync with 'go mod tidy'"
+		exit 255
+	fi
+}
+
 for pass in $PASSES; do
 	echo "Starting '$pass' pass at $(date)"
 	"${pass}"_pass "$@"