[libcxx] Make the GDB pretty printer test less strict

[phosek]

Summary

This is a workaround for PR48937. GDB can sometimes print additional
warnings which currently fails the test. Use re.search instead of
re.match to ignore this additional output.

[phosek]

I think it'd be better to actually address the RTTI issue in PR48937, but this can be used as a temporary workaround.

[phosek]

@ldionne @curdeius friendly ping?

[ldionne]

I defer my review to the author of those pretty printers.

libcxx/test/libcxx/gdb/gdb_pretty_printer_test.py

def invoke(self, arg, from_tty):
 
             # Ignore the convenience variable name and newline
             value = value_str[value_str.find("= ") + 2:-1]
             gdb.newest_frame().select()
 
             expectation_val = compare_frame.read_var("expectation")
             check_literal = expectation_val.string(encoding="utf-8")
             if "PrettyPrintToRegex" in compare_frame.name():
-                test_fails = not re.match(check_literal, value)
+                test_fails = not re.search(check_literal, value)
             else:
                 test_fails = value != check_literal
 
             if test_fails:
                 global test_failures
                 print("FAIL: " + test_loc.symtab.filename +
                       ":" + str(test_loc.line))
                 print("GDB printed:")