Make lots of tests run in parallel

Putting t.Parallel() in each test makes them run in parallel.
Additional t.Parallel() could be added to each subtest, although
that requires making a local copy of the loop variable for
table driven tests.

Test: m checkbuild
Change-Id: I5d9869ead441093f4d7c5757f2447385333a95a4

cmd/zip2zip/zip2zip_test.go

@@ -416,6 +416,7 @@ func errorString(e error) string {
 }
 
 func TestZip2Zip(t *testing.T) {
+	t.Parallel()
 	for _, testCase := range testCases {
 		t.Run(testCase.name, func(t *testing.T) {
 			inputBuf := &bytes.Buffer{}
@@ -472,6 +473,7 @@ func TestZip2Zip(t *testing.T) {
 }
 
 func TestConstantPartOfPattern(t *testing.T) {
+	t.Parallel()
 	testCases := []struct{ in, out string }{
 		{
 			in:  "",