Fix macro_assembler.nim and re-enable tests

[kdeme]

In #1445 3 tests got disabled: test_op_env, test_op_misc and test_txpool.
This is because they would fail on the usage of the Stop op code.

It seems that the macro breaks on an ambiguous identifier for Stop. There is ambiguity between the Stop from the Ops enum and the Stop from the ServerCommand enum in chronos. (The file where the macro is located is already not importing the one of Chronos: https://github.com/status-im/nimbus-eth1/blob/master/tests/macro_assembler.nim#L17. But that is likely not enough and it is probably due to the imports of test where the macro is used).

Some possible fixes:

• Figure out where the leakage comes from and try to exclude ServerCommand from (I've tried this to some degree but can stop the leaking so far, and it is definitely not from an export).
• Adjust the macro to deal with the nnkClosedSymChoice and select the right one.
• Don't work with the untyped opcodes but instead work with strings that are then converted with parseEnum.

[zah]

Please note that the third option does not require changing the syntax in any way. It's merely a suggestion to immediately turn the identifier into a string within the macro and then to use parseEnum on that string instead of using any of the more complicated getType machinery.