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
diff --git a/cmd/diff_target_files/compare_test.go b/cmd/diff_target_files/compare_test.go
index 9d3f8a5..7621246 100644
--- a/cmd/diff_target_files/compare_test.go
+++ b/cmd/diff_target_files/compare_test.go
@@ -21,6 +21,7 @@
 )
 
 func TestDiffTargetFilesLists(t *testing.T) {
+	t.Parallel()
 	zipArtifactFile := func(name string, crc32 uint32, size uint64) *ZipArtifactFile {
 		return &ZipArtifactFile{
 			File: &zip.File{