[fenix] Closes https://github.com/mozilla-mobile/fenix/issues/18102: remove intermittent failing App onCreate duration measurement test.

The test as it exists relies on the robolectric lifecycle, which is hard
to predict, so it doesn't seem worth fixing the test. Writing the test
any other way would require excessive mocking, which also seems
impractical.

app/src/test/java/org/mozilla/fenix/FenixApplicationTest.kt

@@ -16,15 +16,12 @@ import mozilla.components.feature.addons.migration.DefaultSupportedAddonsChecker
 import mozilla.components.service.glean.testing.GleanTestRule
 import org.junit.Assert.assertEquals
 import org.junit.Assert.assertFalse
-import org.junit.Assert.assertTrue
 import org.junit.Before
-import org.junit.Ignore
 import org.junit.Rule
 import org.junit.Test
 import org.junit.runner.RunWith
 import org.mozilla.fenix.GleanMetrics.Addons
 import org.mozilla.fenix.GleanMetrics.Metrics
-import org.mozilla.fenix.GleanMetrics.PerfStartup
 import org.mozilla.fenix.GleanMetrics.Preferences
 import org.mozilla.fenix.GleanMetrics.SearchDefaultEngine
 import org.mozilla.fenix.components.metrics.MozillaProductDetector
@@ -51,14 +48,6 @@ class FenixApplicationTest {
         browserStore = BrowserStore()
     }
 
-    @Ignore("See https://github.com/mozilla-mobile/fenix/issues/18102")
-    @Test
-    fun `GIVEN onCreate is called THEN the duration is measured`() {
-        // application.onCreate is called before the test as part of test set up:
-        // https://robolectric.blogspot.com/2013/04/the-test-lifecycle-in-20.html
-        assertTrue(PerfStartup.applicationOnCreate.testHasValue())
-    }
-
     @Test
     fun `GIVEN there are unsupported addons installed WHEN subscribing for new add-ons checks THEN register for checks`() {
         val checker = mockk<DefaultSupportedAddonsChecker>(relaxed = true)