test: write heap dump to log folder

Per default the heap dump is written to target/JX/pidXYZ.hprof
In order to keep them when a new test is is started, they
should be written to log folder which is not cleared in a new
test run.
Heap dump location can be set with -Dtests.heapdump.path=/path/to/heapdump

closes #7452
Validate create index requests' number of primary/replica shards

Fixes #7495
[TEST] apply default settings before test specific ones to external nodes in bw comp tests, otherwise the defaults win all the time
[TEST] make default settings don't override test specific settings
[TEST] apply default settings by calling super.nodeSettings method when providing test specific methods