Improve test coverage for codebase

[jonhoo]

The code coverage of our tests is good, but not as good as we'd like it to be. In particular, I see some obvious candidates for new tests.

In perf stack collapsing (see coverage report):

• Collapsing a perf trace with an empty line.
• Collapsing a perf trace with a "weird" event line.
• Collapsing a perf trace with a "process name".
• Collapsing a perf trace with an inline Java function.
• Collapsing a perf trace with a "weird" stack line.
• Collapse a profile with Go names.

In DTrace stack collapsing (see coverage report):

• Collapse a file with only header lines.
• Collapse a file with a line that has ::, but not a trailing argument list.

Stack collapsing in general:

• Collapse using collapse_file.
• Collapse using collapse_file on STDIN.

In flamegraph (see coverage report):

• Use a palette file.
• Ignore bad lines.
• Plot an empty trace.
• Plot a file where frames get pruned.
• Plot an inverted flame graph.
• Include grey frames.
• Test a non-sorted multi-reader input.
• Add tests for all the various color mappings.
• Add unit tests for Java semantic coloring.
• Add unit tests for Perl semantic coloring.
• Add unit tests for JavaScript semantic coloring.
• Add tests for various malformed palette lines (see coverage).
• Add a test that has to generate colors (you may need to enable hash).
• Add tests for various malformed nameattribute lines.

[jonhoo]

It'd also be really cool if someone fuzzed inferno to see that we don't have any truly silly crashes!