REGTEST: replace LEVEL option by a more human readable one.

This patch replaces LEVEL variable by REGTESTS_TYPES variable which is more
mnemonic and human readable. It is uses as a filter to run the reg tests scripts
where a commented #REGTEST_TYPE may be defined to designate their types.
Running the following command:

    $ REGTESTS_TYPES=slow,default

will start all the reg tests where REGTEST_TYPE is defines as 'slow' or 'default'.
Note that 'default' is also the default value of REGTEST_TYPE when not specified
dedicated to run all the current h*.vtc files. When REGTESTS_TYPES is not specified
there is no filter at all. All the tests are run.

This patches also defines REGTEST_TYPE with 'slow' value for all the s*.vtc files,
'bug' value for al the b*.vtc files, 'broken' value for all the k*.vtc files.
diff --git a/reg-tests/checks/s00001.vtc b/reg-tests/checks/s00001.vtc
index 2e38867..b37e868 100644
--- a/reg-tests/checks/s00001.vtc
+++ b/reg-tests/checks/s00001.vtc
@@ -8,6 +8,7 @@
 
 #REQUIRE_VERSION=1.8
 #EXCLUDE_TARGETS=freebsd
+#REGTEST_TYPE=slow
 
 # Note that the first syslog message received is: "Proxy <name> started."
 syslog S -repeat 21 -level notice {