test/scylla-gdb: detect Scylla compiled without debugging information
test/scylla-gdb: detect Scylla compiled without debugging information

test/scylla-gdb tests Scylla's gdb debugging tools, and cannot work if
Scylla was compiled without debug information (i.e, the "dev" build mode).
In the past, test/scylla-gdb/run detected this case and printed a clear error:

   Scylla executable was compiled without debugging information (-g)
   so cannot be used to test gdb. Please set SCYLLA environment variable.

Unfortunately, since recently this detection fails, because even when
Scylla is compiled without debug information we link into it a library
(libwasmtime.a) which has *some* debug information. As a result, instead
of one clear error message, we get all scylla-gdb tests running -
and each of them failing separately. This is ugly and unhelpful.

Each of the tests fail because our "gdb" test fixture tries to load
scylla-gdb.py and fails when the symbols it needs (e.g., "size_t")
cannot be found. So in this patch, we check once for the existance
of this symbol - and if missing we exit pytest instead of failing each
individual test.

Moreover, if loading scylla-gdb.py fails for some other unexpected
reason, let's exit the test as well, instead of failing each individual
test.

Fixes #10863.

Signed-off-by: Nadav Har'El <nyh@scylladb.com>

Closes #10937

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla