Replace file reading code in wxTextFile with wxFile::ReadAll()

There is nothing special about wxTextFile justifying having code for
dealing with non-seekable files in it, so put this code into wxFile
itself and just call its ReadAll() method from here.

This is a better fix than 41f6f17d01
originally applied (and which is going to be reverted next) as it
doesn't break wxFile::Length() for these files and also avoids
triggering an assert if the file we're trying to read was truncated by
another process in the meanwhile -- which can happen and doesn't
indicate a programming error and so shouldn't result in an assert.

Also add a unit test checking that this really works.

See #3802, #8354, #9965.

tests/textfile/textfiletest.cpp

@@ -338,5 +338,29 @@ void TextFileTestCase::ReadBig()
                           f[NUM_LINES - 1] );
 }
 
-#endif // wxUSE_TEXTFILE
+#ifdef __LINUX__
 
+// Check if using wxTextFile with special files, whose reported size doesn't
+// correspond to the real amount of data in them, works.
+TEST_CASE("wxTextFile::Special", "[textfile][linux][special-file]")
+{
+    SECTION("/proc")
+    {
+        wxTextFile f;
+        CHECK( f.Open("/proc/diskstats") );
+        CHECK( f.GetLineCount() > 1 );
+    }
+
+    SECTION("/sys")
+    {
+        wxTextFile f;
+        CHECK( f.Open("/sys/power/state") );
+        REQUIRE( f.GetLineCount() == 1 );
+        INFO( "/sys/power/state contains \"" << f[0] << "\"" );
+        CHECK( f[0].find("mem") != wxString::npos );
+    }
+}
+
+#endif // __LINUX__
+
+#endif // wxUSE_TEXTFILE