Fix the gcovr coverage regex by removing line separators before scanning

RE2 differs from Ruby in handling multiple-line strings. The string "foo\n"
will not match the regular expression "foo$" unless multi-line mode is enabled
(and it's off by default).

Since we're already scanning the build trace line by line (and so multi-line
coverage regular expressions won't work), we can fix this by removing the line
separator before scanning the string.

lib/gitlab/ci/trace/stream.rb

@@ -74,6 +74,7 @@ module Gitlab
           match = ""
 
           reverse_line do |line|
+            line.chomp!
             matches = regex.scan(line)
             next unless matches.is_a?(Array)
             next if matches.empty?

spec/lib/gitlab/ci/trace/stream_spec.rb

@@ -300,5 +300,12 @@ describe Gitlab::Ci::Trace::Stream do
 
       include_examples 'malicious regexp'
     end
+
+    context 'multi-line data with rooted regexp' do
+      let(:data) { "\n65%\n" }
+      let(:regex) { '^(\d+)\%$' }
+
+      it { is_expected.to eq('65') }
+    end
   end
 end