Use BigMath.E to allow running test in isolation

* BigMath.exp may be defined in another file, this then fails
  bin/jruby -S rake test:mri TEST=test/mri/bigdecimal/test_bigdecimal.rb
Add a high-precision test (50 digits BigMath.log)

* excluded the test by default since it slows down the
  tests by 0.2 seconds