testutils: allow overriding DefaultTestDRPCOption with env variable

This allows enabling or disabling DRPC for unit tests without changing code
or recompiling. Note that the environment variable will take precedence
over server args, which take precedence over the package-level option.

Example usage: `dev test pkg/server:server_test -- --test_env=COCKROACH_TEST_DRPC=false`

Epic: none
Release note: none

Author: shubhamdhama

pkg/testutils/serverutils/test_server_shim.go

@@ -177,6 +177,13 @@ const (
 
 	testTenantModeEnabledShared   = "shared"
 	testTenantModeEnabledExternal = "external"
+
+	// COCKROACH_TEST_DRPC controls the DRPC enablement mode for test servers.
+	//
+	// - disabled: disables DRPC; all inter-node connectivity will use gRPC only
+	//
+	// - enabled: enables DRPC for inter-node connectivity
+	testDRPCEnabledEnvVar = "COCKROACH_TEST_DRPC"
 )
 
 func testTenantDecisionFromEnvironment(
@@ -537,17 +544,41 @@ func WaitForTenantCapabilities(
 	}
 }
 
+// parseDefaultTestDRPCOptionFromEnv parses the COCKROACH_TEST_DRPC environment
+// variable and returns the corresponding DefaultTestDRPCOption. If the
+// environment variable is not set it returns TestDRPCUnset. For invalid value,
+// it panic.
+func parseDefaultTestDRPCOptionFromEnv() base.DefaultTestDRPCOption {
+	if str, present := envutil.EnvString(testDRPCEnabledEnvVar, 0); present {
+		switch str {
+		case "disabled", "false":
+			return base.TestDRPCDisabled
+		case "enabled", "true":
+			return base.TestDRPCEnabled
+		default:
+			panic(fmt.Sprintf("invalid value for %s: %s", testDRPCEnabledEnvVar, str))
+		}
+	}
+	return base.TestDRPCUnset
+}
+
 // ShouldEnableDRPC determines the final DRPC option based on the input
 // option and any global overrides, resolving random choices to a concrete
 // enabled/disabled state.
 func ShouldEnableDRPC(
 	ctx context.Context, t TestLogger, option base.DefaultTestDRPCOption,
 ) base.DefaultTestDRPCOption {
 	var logSuffix string
-	if option == base.TestDRPCUnset && globalDefaultDRPCOptionOverride.isSet {
+
+	// Check environment variable first
+	if envOption := parseDefaultTestDRPCOptionFromEnv(); envOption != base.TestDRPCUnset {
+		option = envOption
+		logSuffix = " (override by COCKROACH_TEST_DRPC environment variable)"
+	} else if option == base.TestDRPCUnset && globalDefaultDRPCOptionOverride.isSet {
 		option = globalDefaultDRPCOptionOverride.value
 		logSuffix = " (override by TestingGlobalDRPCOption)"
 	}
+
 	enableDRPC := false
 	switch option {
 	case base.TestDRPCEnabled: