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/log/b00000.vtc b/reg-tests/log/b00000.vtc
index bd4ba0f..228c3fd 100644
--- a/reg-tests/log/b00000.vtc
+++ b/reg-tests/log/b00000.vtc
@@ -18,6 +18,8 @@
 # Note that the problem doesn't happen when using %[src] or %[dst] since
 # all sample expressions set LW_XPRT.
 
+#REGTEST_TYPE=bug
+
 varnishtest "Wrong ip/port logging"
 feature ignore_unknown_macro