test_bigmath.rb: test_log

* test/bigdecimal/test_bigmath.rb (test_log): add basic test for BigMath.log.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44560 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
test_bigmath.rb: r44558

* test/bigdecimal/test_bigmath.rb (test_log): add assertion for r44558.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44561 b2dd03c8-39d4-4d8f-98ff-823fe69b080e