[lldb] [testsuite] Fix false failures on "\n" regex match

Regex is matching a binary so it needs `re.DOTALL`.
vFile:fstat packet data ATTACHMENT is not hex-encoded but it is only
escaped.

It is difficult to reproduce, it came from buildbot:
  https://lab.llvm.org/staging/#/builders/16/builds/10597

******************** TEST 'lldb-api :: tools/lldb-server/TestGdbRemotePlatformFile.py' FAILED ********************
FAIL: test_platform_file_fstat_llgs (TestGdbRemotePlatformFile.TestGdbRemotePlatformFile)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "lldb/packages/Python/lldbsuite/test/tools/lldb-server/gdbremote_testcase.py", line 52, in test_method
    return attrvalue(self)
  File "lldb/test/API/tools/lldb-server/TestGdbRemotePlatformFile.py", line 259, in test_platform_file_fstat
    context = self.expect_gdbremote_sequence()
  File "lldb/packages/Python/lldbsuite/test/tools/lldb-server/gdbremote_testcase.py", line 621, in expect_gdbremote_sequence
    return expect_lldb_gdbserver_replay(
  File "lldb/packages/Python/lldbsuite/test/tools/lldb-server/lldbgdbserverutils.py", line 198, in expect_lldb_gdbserver_replay
    context = sequence_entry.assert_match(
  File "lldb/packages/Python/lldbsuite/test/tools/lldb-server/lldbgdbserverutils.py", line 479, in assert_match
    return self._assert_regex_match(asserter, actual_packet, context)
  File "lldb/packages/Python/lldbsuite/test/tools/lldb-server/lldbgdbserverutils.py", line 446, in _assert_regex_match
    asserter.fail(
AssertionError: regex '^\$F([0-9a-fA-F]+);(.*)#[0-9a-fA-F]{2}$' failed to match against content '$F40;^@^@^@ ^@
^C<8d>^@^@<81><80>^@^@^@^A^@^@^CX^@^@^CX^@^@^@^@^@^@^@^@^@^@^@^W^@^@^@^@^@^@^P^@^@^@^@^@^@^@^@
a=XXa=XXa=XX#6b'