[gdb/testsuite] Fix FAILs due to verbose in foll-fork.exp

[gdb/testsuite] Fix FAILs due to verbose in foll-fork.exp

When running test-case gdb.base/foll-fork.exp, I see:
...
(gdb) catch fork^M
Catchpoint 2 (fork)^M
Reading in symbols for ../sysdeps/x86/libc-start.c...^M
(gdb) FAIL: gdb.base/foll-fork.exp: explicit child follow, set catch fork
...

The problem is that the test regexp does not expect the "Reading in symbols"
message:
...
    gdb_test "catch fork" "Catchpoint \[0-9\]* \\(fork\\)" \
        "explicit child follow, set catch fork"
...
which is generated due to the verbose setting.

Fix this by allowing the message in the regexp.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-03-11  Tom de Vries  <tdevries@suse.de>

	* gdb.base/foll-fork.exp: Allow "Reading in symbols" messages.