Ensure main file has default coverage if required. (#7169)

	* Extract common code for coverage setup.
	---
	 iseq.c | 13 +++++++++++--
	 1 file changed, 11 insertions(+), 2 deletions(-)

	More coverage tests & specs. (#7171)

	* Add spec for eval and line coverage.

	* Add test for main file coverage.
	---
	 spec/ruby/library/coverage/start_spec.rb | 8 +++++++-
	 test/coverage/autostart.rb               | 2 ++
	 test/coverage/main.rb                    | 1 +
	 test/coverage/test_coverage.rb           | 7 +++++++
	 4 files changed, 17 insertions(+), 1 deletion(-)
	 create mode 100644 test/coverage/autostart.rb
	 create mode 100644 test/coverage/main.rb