tests: appsrc: clean up block_deadlock test and make it work in valgrind

tests: appsrc: clean up block_deadlock test and make it work in valgrind

Remove all the bus watch and main loop code from the block_deadlock
test, it's not needed: neither pipeline will ever post an EOS or ERROR
message on the bus, and we're the only ones posting an error, from a
timeout. Might just as well just sleep for a bit and then do whatever
we want to do.

Don't gratuitiously set tcase timeout, just use whatever is the
default (or set via the environment).

Make individual pipeline runs shorter.

Check for valgrind and only do a handful iterations when running
in valgrind, not 100 (each iteration takes about 4s on a core i7).

Make videotestsrc output smaller buffers than the default resolution,
we don't care about the buffer contents here anyway.

Fixes test timeouts when run in valgrind.