kvserver: test environment variable to disble async intent resolution

This commit adds a new test-only environment variable that disables
async intent resolution (similarly to the
`DisableAsyncIntentResolution` testing knob). This new variable will be
useful in writing roachtests for the vistualized intent resolution
work.

Part of: #156800

Release note: None

Author: miraradeva

pkg/kv/kvserver/intentresolver/intent_resolver.go

@@ -35,6 +35,13 @@ import (
 	"github.com/cockroachdb/errors"
 )
 
+// asyncIntentResolutionDisabled disables asynchronous intent resolution. It is
+// similar to DisableAsyncIntentResolution in IntentResolverTestingKnobs, but
+// for the use of roachtests.
+var asyncIntentResolutionDisabled = envutil.EnvOrDefaultBool(
+	"COCKROACH_TEST_ONLY_ASYNC_INTENT_RESOLUTION_DISABLED", false,
+)
+
 // defaultTaskLimit is the maximum number of asynchronous tasks that may be
 // started by intentResolver. When this limit is reached asynchronous tasks will
 // start to block to apply backpressure.  This is a last line of defense against
@@ -491,7 +498,7 @@ func (ir *IntentResolver) MaybePushTransactions(
 func (ir *IntentResolver) runAsyncTask(
 	origCtx context.Context, allowSyncProcessing bool, taskFn func(context.Context),
 ) error {
-	if ir.testingKnobs.DisableAsyncIntentResolution {
+	if ir.testingKnobs.DisableAsyncIntentResolution || asyncIntentResolutionDisabled {
 		return errors.New("intents not processed as async resolution is disabled")
 	}
 	ctx, hdl, err := ir.stopper.GetHandle(