runtime: increase repeat count for alloc test

To make sure a single spurious alloc doesn't flake the test.

Fixes #75858

Change-Id: I055b37ad5668459bfa7ab1dac97025c997c68f1a
Reviewed-on: https://go-review.googlesource.com/c/go/+/712201
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@google.com>

Author: randall77

src/runtime/iface_test.go

@@ -60,7 +60,7 @@ func TestCmpIfaceConcreteAlloc(t *testing.T) {
 		t.Skip("skipping on non-gc compiler")
 	}
 
-	n := testing.AllocsPerRun(1, func() {
+	n := testing.AllocsPerRun(100, func() {
 		_ = e == ts
 		_ = i1 == ts
 		_ = e == 1