snippets: add snippet for TestMain

This is a natural extension of the existing test/benchmark functions snippets (tf, bf).

Update #160

Change-Id: I9e787dea7fbcc063b1d1a13dffee6b09905120fe
GitHub-Last-Rev: 3c1f73d
GitHub-Pull-Request: #629
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/254497
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Trust: Suzy Mueller <suzmue@golang.org>

Author: pofl

snippets/go.json

@@ -215,6 +215,11 @@
 			"body": "func Test$1(t *testing.T) {\n\t$0\n}",
 			"description": "Snippet for Test function"
 		},
+		"test main": {
+			"prefix": "tm",
+			"body": "func TestMain(m *testing.M) {\n\t$1\n\n\tos.Exit(m.Run())\n}",
+			"description": "Snippet for TestMain function"
+		},
 		"benchmark function": {
 			"prefix": "bf",
 			"body": "func Benchmark$1(b *testing.B) {\n\tfor ${2:i} := 0; ${2:i} < b.N; ${2:i}++ {\n\t\t$0\n\t}\n}",