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

rust/protobuf_test.go

@@ -20,6 +20,7 @@ import (
 )
 
 func TestRustProtobuf(t *testing.T) {
+	t.Parallel()
 	ctx := testRust(t, `
 		rust_protobuf {
 			name: "librust_proto",