Add support for -type runAll as described in documentation

[geschke]

Hello!

Thanks for this nice MongoDB benchmarking tool!

Just a minor thing — while testing, I noticed an inconsistency in the documentation (README). The implemented parameter is -runAll, but the README mentions -type runAll. When using the latter, it results in an unclear error message:

./mongo-bench -threads 10 -docs 10000  -uri mongodb://<mongodb-url>:27017 -type runAll
2025/03/12 19:47:48 Starting runAll test...
panic: runtime error: index out of range [0] with length 0

goroutine 1 [running]:
main.DocCountTestingStrategy.runTest({}, {0x9f8cb0, 0xc0000561c8}, {0x7ffcbccf453a, 0x6}, {0xa, 0x2710, 0x0, 0x0, 0x1}, ...)
        /home/geschke/tmp/mongodb-bench/docs_testing_strategy.go:172 +0x11f6
main.main()
        /home/geschke/tmp/mongodb-bench/main.go:68 +0x642

The small changes fix the issue so that both the -runAll parameter and -type runAll are supported. Additionally, an error message is now displayed for an unknown -type parameter, which I find more user-friendly than exiting with a panic.

Oh, and the order of the imports was automatically adjusted by Visual Studio Code (or rather, the Go extension). :-)

Thanks again! I'd appreciate if you could merge this PR.

Kind regards,
Ralf