Added explicitly begin-end block for Ruby 2.4.

	  strscan, ipaddr and some default gems still support Ruby 2.4.

	  After this, I extract this CoreAssertions to their repositories.

	Improve base time of assert_linear_performance (#11369)

	Remove `.ceil` from base time calculation that makes 10x gap.
	This will make the assertion more strict and also less flaky.

	Suppress warning for shadowing outer local variable

	core_assertions.rb: Support old rubies

	Some symbol argument might not be accepted by Process.clock_gettime.

	core_assertions.rb: Prefer CPU time clocks

	To prevent influence from other processes.

	core_assertions.rb: Extract common code block

	Skip assert_linear_performance for RJIT

	core_assertions.rb: Tweak timeout limit

	Increase the timeout limit when variance at rehearsal is small.

This reverts commit 367ad5055c3d5210c884d4611e08ffa32ec608ee.

This reverts commit 56c311aa0b20dee13ab43309ae3386f57f8cb797.