Assert that no crashtracker is running before each test

This makes the test fail early if there's an unexpected crash tracker
instance running, rather than failing later in a more confusing way.

spec/datadog/profiling/crashtracker_spec.rb

@@ -4,7 +4,12 @@
 require 'webrick'
 
 RSpec.describe Datadog::Profiling::Crashtracker do
-  before { skip_if_profiling_not_supported(self) }
+  before do
+    skip_if_profiling_not_supported(self)
+
+    crash_tracker_pids = `pgrep -f libdatadog-crashtracking-receiver`
+    expect(crash_tracker_pids).to be_empty, "No crash tracker process should be running, found #{crash_tracker_pids}"
+  end
 
   let(:exporter_configuration) { [:agent, 'http://localhost:6006'] }