Use platform_is guard

Merged: https://github.com/ruby/ruby/pull/2997

spec/ruby/core/file/atime_spec.rb

@@ -15,7 +15,7 @@ describe "File.atime" do
     File.atime(@file).should be_kind_of(Time)
   end
 
-  guard -> { platform_is :linux or platform_is :windows } do
+  platform_is :linux, :windows do
     ## NOTE also that some Linux systems disable atime (e.g. via mount params) for better filesystem speed.
     it "returns the last access time for the named file with microseconds" do
       supports_subseconds = Integer(`stat -c%x '#{__FILE__}'`[/\.(\d+)/, 1], 10)

spec/ruby/core/file/ctime_spec.rb

@@ -14,7 +14,7 @@ describe "File.ctime" do
     File.ctime(@file).should be_kind_of(Time)
   end
 
-  guard -> { platform_is :linux or platform_is :windows } do
+  platform_is :linux, :windows do
     it "returns the change time for the named file (the time at which directory information about the file was changed, not the file itself) with microseconds." do
       supports_subseconds = Integer(`stat -c%z '#{__FILE__}'`[/\.(\d+)/, 1], 10)
       if supports_subseconds != 0

spec/ruby/core/file/mtime_spec.rb

@@ -15,7 +15,7 @@ describe "File.mtime" do
     File.mtime(@filename).should be_close(@mtime, TIME_TOLERANCE)
   end
 
-  guard -> { platform_is :linux or platform_is :windows } do
+  platform_is :linux, :windows do
     it "returns the modification Time of the file with microseconds" do
       supports_subseconds = Integer(`stat -c%y '#{__FILE__}'`[/\.(\d+)/, 1], 10)
       if supports_subseconds != 0

spec/ruby/core/file/utime_spec.rb

@@ -3,7 +3,7 @@ require_relative '../../spec_helper'
 describe "File.utime" do
 
   before :all do
-    @time_is_float = /mswin|mingw/ =~ RUBY_PLATFORM
+    @time_is_float = platform_is :windows
   end
 
   before :each do