REGTEST/MINOR: Add levels to reg-tests target.

REGTEST/MINOR: Add levels to reg-tests target.

With this patch we can provide LEVEL environment variable when
running reg-tests Makefile targe (reg testing) to set the execution
level of the reg-tests make target to run.

LEVEL default value is 1.

LEVEL=1 is to run all h*.vtc files which are the most important
reg testing files (to test haproxy core, HTTP compliance etc).

LEVEL=2 is to run all s*.vtc files which are a bit slow tests,
for instance tests requiring external programs (curl, socat etc).

LEVEL=3 is to run all l*.vtc files which are test files with again
more slow or with little interest.