fix testmempoolaccept CLI syntax
fix testmempoolaccept CLI syntax

`testmempoolaccept "hexstring"` will give a "JSON parse error". The correct syntax is `testmempoolaccept \[\"hexstring\"\]` (but seems escaping is not displayed in other areas so leaving backspaces out).

Github-Pull: #14966
Rebased-From: b74a52192bcefea6fdc38f244e9500ce5868a0b5

qtumproject/qtum,qtumproject/qtum,GroestlCoin/bitcoin,guncoin/guncoin,GroestlCoin/GroestlCoin,GroestlCoin/bitcoin,qtumproject/qtum,GroestlCoin/GroestlCoin,lbryio/lbrycrd,lbryio/lbrycrd,qtumproject/qtum,qtumproject/qtum,GroestlCoin/GroestlCoin,GroestlCoin/bitcoin,guncoin/guncoin,GroestlCoin/GroestlCoin,guncoin/guncoin,lbryio/lbrycrd,qtumproject/qtum,GroestlCoin/bitcoin,lbryio/lbrycrd,lbryio/lbrycrd,guncoin/guncoin,GroestlCoin/GroestlCoin,lbryio/lbrycrd,guncoin/guncoin,lbryio/lbrycrd,GroestlCoin/GroestlCoin,guncoin/guncoin,GroestlCoin/bitcoin,qtumproject/qtum,GroestlCoin/bitcoin