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/merge_zips/merge_zips_test.go

@@ -85,6 +85,7 @@ func (tiz *testInputZip) IsOpen() bool {
 }
 
 func TestMergeZips(t *testing.T) {
+	t.Parallel()
 	testCases := []struct {
 		name             string
 		in               [][]testZipEntry
@@ -361,6 +362,7 @@ func (diz *DummyInpuZip) IsOpen() bool {
 }
 
 func TestInputZipsManager(t *testing.T) {
+	t.Parallel()
 	const nInputZips = 20
 	const nMaxOpenZips = 10
 	izm := NewInputZipsManager(20, 10)