Restore original System Properties after tests

Many registry tools tests modify system properties but do not
restore them to original state. These tests must be isolated
from each other and cannot share the same test execution process.

This has a huge impact on test performance under Gradle, which
seems to have higher process startup overhead. Current Gradle
test config has to set 'forEvery' to 1, i.e., every test class
must be run in a freshly started process.

This change significantly reduces the number of tests that need
isolation, making it easier to optimize test config for the
remaining tests.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=221350284

java/google/registry/config/RegistryEnvironment.java

@@ -47,11 +47,25 @@ public enum RegistryEnvironment {
    */
   UNITTEST;
 
+  /** Sets this enum as the name of the registry environment. */
+  public RegistryEnvironment setup() {
+    return setup(SystemPropertySetter.PRODUCTION_IMPL);
+  }
+
+  /**
+   * Sets this enum as the name of the registry environment using specified {@link
+   * SystemPropertySetter}.
+   */
+  public RegistryEnvironment setup(SystemPropertySetter systemPropertySetter) {
+    systemPropertySetter.setProperty(PROPERTY, name());
+    return this;
+  }
+
   /** Returns environment configured by system property {@value #PROPERTY}. */
   public static RegistryEnvironment get() {
     return valueOf(Ascii.toUpperCase(System.getProperty(PROPERTY, UNITTEST.name())));
   }
 
   /** System property for configuring which environment we should use. */
-  public static final String PROPERTY = "google.registry.environment";
+  private static final String PROPERTY = "google.registry.environment";
 }