Make parse_static_tracepoint_marker_definition work with multiple static tracepoint...

Make parse_static_tracepoint_marker_definition work with multiple static tracepoint definitions

Since I modify the parse_static_tracepoint_marker_definition function in
the next patch, I wanted to write a unit test for it.  Doing so showed
that it doesn't handle multiple consecutive static tracepoint
definitions separated by commas.  However, the RSP documentation [1]
states that servers may return multiple definitions, like:

  1234:6d61726b657231:6578747261207374756666,abba:6d61726b657232:

The problem is that the function uses strlen to compute the length of
the last field (the extra field).  If there are additional definitions
in addition to the one we are currently parsing, the returned length
will include those definitions, and we'll try to hex-decode past the
extra field.

This patch changes parse_static_tracepoint_marker_definition to consider
the case where the current definition is followed by a comma and more
definitions.  It also adds the unit test that found the issue in the
first place.

I don't think this causes any backwards compatibility issues, because
the previous code only handled single static tracepoint definitions, and
the new code handles that correctly.

gdb/ChangeLog:

	* tracepoint.c (parse_static_tracepoint_marker_definition):
	Consider case where the definition is followed by more
	definitions.
	* Makefile.in (SUBDIR_UNITTESTS_SRCS): Add
	tracepoint-selftests.c.
	* unittests/tracepoint-selftests.c: New.

[1] https://sourceware.org/gdb/onlinedocs/gdb/Tracepoint-Packets.html#qTfSTM