cobertura: emit line and branch coverage for each method

[RoadTrain]

Cobertura is usually structured in way that line coverage is presented for both classes and methods, i.e. each method definition contains line coverage for code within a method, while a class definition contain line coverage for the entire class. Currently istanbuljs emits only 1 line per method (method declaration) while correctly emitting coverage for classes.

See danielpalme/ReportGenerator#603 for more context.

This PR improves Cobertura generation so that for each method there is separate line coverage data.

[RoadTrain]

TODO: Fix tests

[bcoe]

@RoadTrain I have never used the Cobertura format myself and am open to suggestions if you can get tests passing. I would also like to make sure that we don't break tests in major dependencies like c8 and nyc.

[RoadTrain]

A quick update. I will try to get back to it soon. Some tests were broken even before my changes, so I have to makes sure they are fixed in a correct way.

[RoadTrain]

For future reference, there's an official Cobertura spec, which specifies line data for methods: https://github.com/cobertura/cobertura/blob/master/cobertura/src/site/htdocs/xml/coverage-04.dtd

The original cobertura tool (for java projects) emits methods with all line data corresponding to each particular method: https://github.com/cobertura/cobertura/blob/0ff963284cecaace30f409a977dccb07c41a5a8f/cobertura/src/main/java/net/sourceforge/cobertura/reporting/xml/XMLReport.java#L215

C# coverlet cobertura reporter does the same: https://github.com/coverlet-coverage/coverlet/blob/cceb71d758a57425ef54ca6bf9f0e443dfe378c5/src/coverlet.core/Reporters/CoberturaReporter.cs#L125

So this PR is basically bringing instanbuljs cobertura reporter in line with other reporters.

Hopefully I can get it finished after tests are fixed (#748).