Measure the test coverage without SimpleCov

Now `make test-all COVERAGE=true` measures the test coverage by using
`coverage.so` directly, and visualize the result by simplecov-html.

There has been some problems in coverage measurement with SimpleCov.
(They are not SimpleCov's fault, though.)

(1) It is difficult to extract the measured data as a machine-readable
format, such as Marshal.  I want to visualize the coverage data with
other coverage tools, such as LCOV and Cobertura.
(I know we can use SimpleCov's formatter mechanism, but I don't want
to depend upon SimpleCov so much.)

(2) SimpleCov seems to miss some coverage data.  For example,
`lib/cgi.rb` and `lib/ostruct.rb` are dropped.  I don't know why.

(3) I have a plan to enhance `coverage.so` with branch coverage.
It would be difficult to continue to only use SimpleCov as is.
This is the most important reason.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59770 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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