tests: Fix tst_qtextcodec to start echo_helper from test directory

Task-number: QTBUG-118680
Change-Id: I80f7b78143de8ae6347b0b3b74456c3de899b779
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

tests/auto/core5/codecs/qtextcodec/tst_qtextcodec.cpp

@@ -2239,7 +2239,7 @@ void tst_QTextCodec::toLocal8Bit()
     QSKIP("No qprocess support", SkipAll);
 #else
     QProcess process;
-    process.start("echo_helper");
+    process.start(QFINDTESTDATA("echo_helper"));
     QString string(QChar(0x410));
     process.write((const char*)string.utf16(), string.size()*2);