[lldb] [test] Fix test_platform_file_fstat to account for negative ints

[mgorny]

Summary

Fix test_platform_file_fstat to correctly truncate/max out the expected
value when GDB Remote Serial Protocol specifies a value as an unsigned
integer but the underlying platform type uses a signed integer.

Sponsored by: The FreeBSD Foundation

lldb/test/API/tools/lldb-server/TestGdbRemotePlatformFile.py

class GDBStat(typing.NamedTuple):
     st_blksize: int
     st_blocks: int
     st_atime: int
     st_mtime: int
     st_ctime: int
 
 
 def uint32_or_zero(x):
-    return x if x < 2**32 else 0
+    return x if x < 2**32 and x >= 0 else 0
 
 
 def uint32_or_max(x):
-    return x if x < 2**32 else 2**32 - 1
+    return x if x < 2**32 and x >= 0 else 2**32 - 1
 
 
 def uint32_trunc(x):
     return x & (2**32 - 1)
 
 
 class TestGdbRemotePlatformFile(GdbRemoteTestCaseBase):