Add more tests for Regexp.linear_time?

> The cache-based optimization now supports lookarounds and atomic groupings. That is, match for Regexp containing these extensions can now also be performed in linear time to the length of the input string. However, these cannot contain captures and cannot be nested. [Feature #19725]

Author: trinistr

core/regexp/linear_time_spec.rb

@@ -25,7 +25,27 @@
     }.should complain(/warning: flags ignored/)
   end
 
-  it "returns true for positive lookarounds" do
-    Regexp.linear_time?(/(?:(?=a*)a)*/).should == true
+  it "returns true for positive lookahead" do
+    Regexp.linear_time?(/a*(?:(?=a*)a)*b/).should == true
+  end
+
+  it "returns true for positive lookbehind" do
+    Regexp.linear_time?(/a*(?:(?<=a)a*)*b/).should == true
+  end
+
+  it "returns true for negative lookahead" do
+    Regexp.linear_time?(/a*(?:(?!a*)a*)*b/).should == true
+  end
+
+  it "returns true for negative lookbehind" do
+    Regexp.linear_time?(/a*(?:(?<!a)a*)*b/).should == true
+  end
+
+  it "returns true for atomic groups" do
+    Regexp.linear_time?(/a*(?:(?>a)a*)*b/).should == true
+  end
+
+  it "returns true for possessive quantifiers" do
+    Regexp.linear_time?(/a*(?:(?:a)?+a*)*b/).should == true
   end
 end