"code" here is too ambiguous. TracePoint events only occur if there
is a new statement or expression on that line, not if the line
is a continuation of a previous statement or expression and there
is no new statement or expression on the line.
For example:
```
[
foo, # start of expression, line event
bar # continuation of expression, no line event
]
[
foo, # start of expression, line event
(bar) # new expression, line event
]
foo( # start of expression, line event
bar # continuation of expression, no line event
)
foo( # start of expression, line event
(bar) # new expression, line event
)
```
Fixes [Bug #15634]
* Fix debug documents to match Thread#to_s change (Feature #16412 ticket)
* TracePoint#inspect returns "... file:line" (Feature #16513)
* Guard older version of Ruby in Tracepoint inspection tests
* Focus on current thread only when running TracePoint inspection test
