Add t.Parallel() to SDK tests

Reduces time to run go test ./sdk from 44 seconds to 22.5 seconds.

Requires moving snapshotTestCustomizations from a map to a struct
with individual fields to avoid concurrent map accesses between
subtests.

Test: go test ./sdk
Change-Id: Id6f451ba9cbace8bc7ea915033a795456b85cf3f
diff --git a/sdk/exports_test.go b/sdk/exports_test.go
index 9d0a242..5a7ce84 100644
--- a/sdk/exports_test.go
+++ b/sdk/exports_test.go
@@ -20,6 +20,7 @@
 
 // Ensure that module_exports generates a module_exports_snapshot module.
 func TestModuleExportsSnapshot(t *testing.T) {
+	t.Parallel()
 	packageBp := `
 		module_exports {
 			name: "myexports",