[core] Add TROOT::AddExtraInterpreterArgs():

Some of the tests need to find headers at runtime. Previously, these headers
were found by their full path; now, ROOT_GENERATE_DICTIONARY prefers relative
paths, that make dictionaries relocatable.
To not force everyone to define ROOT_INCLUDE_PATH when running e.g. a stress
test, the binaries call TROOT::AddExtraInterpreterArgs() passing the location
of the header, at runtime, such that cling can find them.

This is possible because the interpreter is constructed lazily, not during
static init; providing the extra interpreter flags early in main() is thus
sufficient.