spec/ruby/core/time/: Use near time for timezone test

`time_with_zone.zone.should == (time_with_zone - 60*60).zone` fails when
the time is immediately before the change of summer time.

https://rubyci.org/logs/rubyci.s3.amazonaws.com/unstable11s/ruby-master/log/20200328T232504Z.fail.html.gz

```
1)
Time#+ preserves time zone FAILED
Expected "CET" == "CEST"
to be truthy but was false
```

It is acceptable as it fails at most twice per year, but it would be
good to use near time objects to reduce the possibility.

core/time/minus_spec.rb

@@ -92,17 +92,17 @@
 
   it "preserves time zone" do
     time_with_zone = Time.now.utc
-    time_with_zone.zone.should == (time_with_zone - 60*60).zone
+    time_with_zone.zone.should == (time_with_zone - 1).zone
 
     time_with_zone = Time.now
-    time_with_zone.zone.should == (time_with_zone - 60*60).zone
+    time_with_zone.zone.should == (time_with_zone - 1).zone
   end
 
   ruby_version_is "2.6" do
     context "zone is a timezone object" do
       it "preserves time zone" do
         zone = TimeSpecs::Timezone.new(offset: (5*3600+30*60))
-        time = Time.new(2012, 1, 1, 12, 0, 0, zone) - 60*60
+        time = Time.new(2012, 1, 1, 12, 0, 0, zone) - 1
 
         time.zone.should == zone
       end

core/time/plus_spec.rb

@@ -50,17 +50,17 @@
 
   it "preserves time zone" do
     time_with_zone = Time.now.utc
-    time_with_zone.zone.should == (time_with_zone + 60*60).zone
+    time_with_zone.zone.should == (time_with_zone + 1).zone
 
     time_with_zone = Time.now
-    time_with_zone.zone.should == (time_with_zone + 60*60).zone
+    time_with_zone.zone.should == (time_with_zone + 1).zone
   end
 
   ruby_version_is "2.6" do
     context "zone is a timezone object" do
       it "preserves time zone" do
         zone = TimeSpecs::Timezone.new(offset: (5*3600+30*60))
-        time = Time.new(2012, 1, 1, 12, 0, 0, zone) + 60*60
+        time = Time.new(2012, 1, 1, 12, 0, 0, zone) + 1
 
         time.zone.should == zone
       end

core/time/succ_spec.rb

@@ -30,7 +30,7 @@
     context "zone is a timezone object" do
       it "preserves time zone" do
         zone = TimeSpecs::Timezone.new(offset: (5*3600+30*60))
-        time = Time.new(2012, 1, 1, 12, 0, 0, zone) - 60*60
+        time = Time.new(2012, 1, 1, 12, 0, 0, zone) - 1
 
         time.zone.should == zone
       end