runtime/pprof: eliminate arbitrary deadline in testCPUProfile
The testCPUProfile helper function iterates until the profile contains
enough samples. However, in general very slow builders may need longer
to complete tests, and may have less-responsive schedulers (leading to
longer durations required to collect profiles with enough samples).
To compensate, slower builders generally run tests with longer timeouts.
Since this test helper already dynamically scales the profile duration
based on the collected samples, allow it to continue to retry and
rescale until it would exceed the test's deadline.