Add initial spec for `Process.clock_gettime`.
user
```ruby
require_relative '../../spec_helper'

describe "Process.clock_gettime" do
  describe 'time units' do
    it 'handles a fixed set of time units' do
      [:nanosecond, :microsecond, :millisecond, :second].each do |unit|
        Process.clock_gettime(Process::CLOCK_MONOTONIC, unit).should be_an_instance_of(Integer)
      end

      [:float_microsecond, :float_millisecond, :float_second].each do |unit|
        Process.clock_gettime(Process::CLOCK_MONOTONIC, unit).should be_an_instance_of(Float)
      end
    end

    it 'raises an ArgumentError for an invalid time unit' do
      lambda { Process.clock_gettime(Process::CLOCK_MONOTONIC, :bad) }.should raise_error(ArgumentError)
    end

    it 'defaults to :float_second' do
      t1 = Process.clock_gettime(Process::CLOCK_MONOTONIC)
      t2 = Process.clock_gettime(Process::CLOCK_MONOTONIC, :float_second)

      t1.should be_an_instance_of(Float)
      t2.should be_close(t1, 2.0)  # 2.0 is chosen arbitrarily to allow for time skew without admitting failure cases, which would be off by an order of magnitude.
    end

    it 'uses the default time unit (:float_second) when passed nil' do
      t1 = Process.clock_gettime(Process::CLOCK_MONOTONIC, nil)
      t2 = Process.clock_gettime(Process::CLOCK_MONOTONIC, :float_second)

      t1.should be_an_instance_of(Float)
      t2.should be_close(t1, 2.0) # 2.0 is chosen arbitrarily to allow for time skew without admitting failure cases, which would be off by an order of magnitude.
    end
  end
end
```
assistant