test_coverage.rb: use real path

* test/coverage/test_coverage.rb (test_restarting_coverage): use
  real directory path for the case $TMPDIR contains symlinks.  on
  macOS, it is defaulted to a path under /var which is a symlink
  to /private/var.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59725 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/coverage/test_coverage.rb

@@ -67,6 +67,7 @@ class TestCoverage < Test::Unit::TestCase
   def test_restarting_coverage
     Dir.mktmpdir {|tmp|
       Dir.chdir(tmp) {
+        tmp = Dir.pwd
         File.open("test.rb", "w") do |f|
           f.puts <<-EOS
             def coverage_test_restarting