Use noncapturing subpattern/parens in gdb_test implementation

Use noncapturing subpattern/parens in gdb_test implementation

This is the portion of gdb_test which performs the match against
the RE (regular expression) passed to it:

    return [gdb_test_multiple $command $message {
        -re "\[\r\n\]*($pattern)\[\r\n\]+$gdb_prompt $" {
            if ![string match "" $message] then {
                pass "$message"
            }
        }

In a test that I've been working on recently, I wanted to use
a backreference - that's the \1 in the the RE below:

gdb_test "info threads"  \
	{.*[\r\n]+\* +([0-9]+) +Thread[^\r\n]* do_something \(n=\1\) at.*}

Put into English, I wanted to make sure that the value of n passed to
do_something() is the same as the thread number shown in the "info
threads" Id column.  (I've structured the test case so that this
*should* be the case.)

It didn't work though.  It turned out that ($pattern) in the RE
noted above is capturing the attempted backreference.  So, in this
case, the backreference does not refer to ([0-9]+) as intended, but
instead refers to ($pattern).  This is wrong because it's not what I
intended, but is also wrong because, if allowed, it could only match a
string of infinite length.

This problem can be fixed by using parens for a "noncapturing
subpattern".  The way that this is done, syntactically, is to use
(?:$pattern) instead of ($pattern).

My research shows that this feature has been present since tcl8.1 which
was released in 1999.

The current tcl version is 8.6 - at least that's what I have on my
machine.  It appears to me that mingw uses some subversion of tcl8.4
which will also have this feature (since 8.4 > 8.1).

So it seems to me that any platform upon which we might wish to test
GDB will have a version of tcl which has this feature.  That being the
case, my hope is that there won't be any objections to its use.

When I looked at the implementation of gdb_test, I wondered whether
the parens were needed at all.  I've concluded that they are.  In the
event that $pattern is an RE which uses alternation at the top level,
e.g. a|b, we need to make $pattern a subpattern (via parens) to limit
the extend of the alternation.  I.e, we don't want the alternation to
extend to the other portions of the RE which gdb_test uses to match
potential blank lines at the beginning of the pattern or the gdb
prompt at the end.

gdb/testsuite/ChangeLog:

	* gdb.exp (gdb_test): Using noncapturing parens for the $pattern
	subpattern.