allow writing to a different runtime log for minitest and clarify readme

Readme.md

@@ -129,7 +129,7 @@ Test groups will often run for different times, making the full test run as slow
 
 **Step 1**: Use these loggers (see below) to record test runtime
 
-**Step 2**: The next test run will use the recorded test runtimes (use `--runtime-log <file>` if you picked a location different from below)
+**Step 2**: The next test run will use the recorded test runtimes (use `--runtime-log <file>` if you wrote to a location different from default)
 
 **Step 3**: Automate upload/download of test runtime from your CI system [example](https://github.com/grosser/parallel_rails_example/blob/master/.github/workflows/test.yml) (chunks need to be combined, an alternative is [amend](https://github.com/grosser/amend))
 
@@ -140,17 +140,17 @@ Rspec: Add to your `.rspec_parallel` (or `.rspec`), but can also be used via `--
     --format progress
     --format ParallelTests::RSpec::RuntimeLogger --out tmp/parallel_runtime_rspec.log
 
-To use a custom logfile location (default: `tmp/parallel_runtime_rspec.log`), use the CLI: `parallel_test spec -t rspec --runtime-log my.log`
-
 ### Minitest
 
 Add to your `test_helper.rb`:
 ```ruby
-require 'parallel_tests/test/runtime_logger' if ENV['RECORD_RUNTIME']
+if ENV['RECORD_RUNTIME']
+   require 'parallel_tests/test/runtime_logger'
+   # ParallelTests::Test::RuntimeLogger.logfile = "tmp/parallel_runtime_test.log" # where to write it
+end
 ```
 
-results will be logged to `tmp/parallel_runtime_test.log` when `RECORD_RUNTIME` is set,
-so it is not always required or overwritten.
+results will be logged when `RECORD_RUNTIME` is set, so it is not always required or overwritten.
 
 Loggers
 =======

lib/parallel_tests/test/runtime_logger.rb

@@ -8,6 +8,8 @@ class RuntimeLogger
       @@prepared = false
 
       class << self
+        attr_writer :logfile
+
         def log_test_run(test)
           prepare
 
@@ -66,7 +68,7 @@ def message(test, delta)
         end
 
         def logfile
-          ParallelTests::Test::Runner.runtime_log
+          @logfile || ParallelTests::Test::Runner.runtime_log
         end
       end
     end

spec/parallel_tests/test/runtime_logger_spec.rb

@@ -51,4 +51,28 @@ def test_foo
       )
     end
   end
+
+  it "can write to a custom location" do
+    skip if RUBY_PLATFORM == "java" # just too slow ...
+    repo_root = Dir.pwd
+
+    use_temporary_directory do
+      FileUtils.mkdir "test"
+      File.write("test/a_test.rb", <<-RUBY)
+        require 'minitest/autorun'
+        require 'parallel_tests/test/runtime_logger'
+        ParallelTests::Test::RuntimeLogger.logfile = "foo.log"
+
+        class Bar < Minitest::Test
+          def test_foo
+            assert true
+          end
+        end
+      RUBY
+
+      run_tests(repo_root)
+
+      expect(File.exist?("foo.log")).to eq(true)
+    end
+  end
 end