test: checks for tracepoint tests

For testing the USDT tracepoint API in the functional tests we
require:
 - that we are on a Linux system*
 - that Bitcoin Core is compiled with tracepoints
 - that bcc and the the Python bcc module [0] is installed
 - that we run the tests with the required permissions**
otherwise we skip the tests.

*:  We currently only support tracepoints on Linux. Tracepoints are
    not compiled on other platforms.
**: Currently, we check for root permissions via getuid == 0. It's
    unclear if it's even possible to run the tests a non-root user
    with e.g. CAP_BPF, CAP_PERFMON, and access to /sys/kernel/debug/
    tracing/. Anyone running these tests as root should carefully
    review them first and then run them in a disposable VM.

[0]: https://github.com/iovisor/bcc/blob/master/INSTALL.md

test/config.ini.in

@@ -25,3 +25,4 @@ RPCAUTH=@abs_top_srcdir@/share/rpcauth/rpcauth.py
 @ENABLE_ZMQ_TRUE@ENABLE_ZMQ=true
 @ENABLE_EXTERNAL_SIGNER_TRUE@ENABLE_EXTERNAL_SIGNER=true
 @ENABLE_SYSCALL_SANDBOX_TRUE@ENABLE_SYSCALL_SANDBOX=true
+@ENABLE_USDT_TRACEPOINTS_TRUE@ENABLE_USDT_TRACEPOINTS=true