[gardening] Reduce number of samples expected in service/get_cpu_samples_rpc_test

This test has been failing on vm-kernel-precomp-win-release-x64 bot as
it was getting only 3-10 samples, while expecting >10.
The expectation doesn't look reliable as we cannot guarantee that
profiler would collect enough samples.
Changed the expectation to >0 samples, in an attempt to make this test
less flaky.

Issue: https://github.com/dart-lang/sdk/issues/43713
Change-Id: I3a8a3854f3828d39eb409b1b540f29a34fe9e6b2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/167043
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>

runtime/observatory/tests/service/get_cpu_samples_rpc_test.dart

@@ -35,7 +35,7 @@ Future checkSamples(Isolate isolate) async {
       reason: "Should have many code objects");
 
   final samples = result['samples'];
-  expect(samples.length, greaterThan(10), reason: "Should have many samples");
+  expect(samples.length, greaterThan(0), reason: "Should have samples");
   final sample = samples.first;
   expect(sample['tid'], isInt);
   expect(sample['timestamp'], isInt);