Fix the Build Log Excerpt Macro Start/End Order

[nwinkler]

The current version of the code has an issue when the end pattern is
matched before the start pattern was found - it simply stops parsing
the log file. This is an issue in case the end pattern appears in the
log multiple times (including before the start pattern), e.g.

Log 1
Log 2
sleep 5
Log 3
Log 4
Run Test
Log 5
Log 6
sleep 5
Log 7
Log 8

If I use the following macro:

${BUILD_LOG_EXCERPT, start="Run Test", end="sleep 5"}

then the current version of the code prints an empty excerpt, since it
stops evaluating the log when it finds the first occurence of sleep 5.
This feels strange, and caused me a couple of lost hours, since I was
trying to figure out why it didn't print anything. Looking at the code,
I found this issue, and have made a small change that ensures that the
end pattern is only checked once the start pattern has been found.

I hope this is fine, but to me it makes a lot more sense like that. I've
also added a unit test to verify that the fix works.

[slide]

Closing and reopening to get a build.