For #13959: add test to prevent increasing StrictMode suppression count.

.github/CODEOWNERS

@@ -48,6 +48,7 @@
 
 # Possible startup regressions
 *Application.kt @mozilla-mobile/Performance
+*StrictMode*kt @mozilla-mobile/Performance
 
 # We want to be aware of new features behind flags as well as features
 # about to be enabled.

app/src/androidTest/java/org/mozilla/fenix/ui/StrictModeStartupSuppressionCountTest.kt

@@ -0,0 +1,57 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+package org.mozilla.fenix.ui
+
+import androidx.test.platform.app.InstrumentationRegistry
+import androidx.test.uiautomator.UiDevice
+import org.junit.Assert.assertEquals
+import org.junit.Rule
+import org.junit.Test
+import org.mozilla.fenix.ext.components
+import org.mozilla.fenix.helpers.HomeActivityTestRule
+
+// PLEASE CONSULT WITH PERF TEAM BEFORE CHANGING THIS VALUE.
+private const val EXPECTED_SUPPRESSION_COUNT = 12
+
+private const val FAILURE_MSG = """StrictMode startup suppression count does not match expected count.
+    Did this PR add or call code that suppresses a StrictMode violation?
+    Did you know that suppressing a StrictMode violation can introduce performance regressions?
+
+    If so, please do your best to implement a solution without suppressing StrictMode.
+    Please consult the perf team if you have questions or believe suppressing StrictMode
+    is the optimal solution.
+
+    If this PR removed code that suppressed StrictMode, great! Please decrement the suppression count.
+
+"""
+
+/**
+ * A performance test to limit the number of StrictMode suppressions on startup.
+ * This test was written by the perf team.
+ *
+ * StrictMode detects main thread IO, which is often indicative of a performance issue.
+ * It's easy to suppress StrictMode so we wrote a test to ensure we have a discussion
+ * if the StrictMode count changes. The perf team is code owners for this file so they
+ * should be notified when the count is modified.
+ *
+ * IF YOU UPDATE THE TEST NAME, UPDATE CODE OWNERS.
+ */
+class StrictModeStartupSuppressionCountTest {
+    @get:Rule
+    val activityTestRule = HomeActivityTestRule(skipOnboarding = true)
+
+    private val uiDevice = UiDevice.getInstance(InstrumentationRegistry.getInstrumentation())
+
+    @Test
+    fun verifyStrictModeSuppressionCount() {
+        uiDevice.waitForIdle() // wait for async UI to load.
+
+        // This might cause intermittents: at an arbitrary point after start up (such as the visual
+        // completeness queue), we might run code on the main thread that suppresses StrictMode,
+        // causing this number to fluctuate depending on device speed. We'll deal with it if it occurs.
+        val actual = activityTestRule.activity.components.strictMode.suppressionCount.toInt()
+        assertEquals(FAILURE_MSG, EXPECTED_SUPPRESSION_COUNT, actual)
+    }
+}