Two tests in rpc_wallet_tests halt the test suite on Windows

[charlieok]

Describe the issue

For these two tests:
rpc_wallet_tests/rpc_wallet_encrypted_wallet_zkeys
rpc_wallet_tests/rpc_wallet_encrypted_wallet_sapzkeys

The tests fail, and cause the test runner to halt.

Can you reliably reproduce the issue?

yes

If so, please list the steps to reproduce below:

1. run a cross-compile on ubuntu to build windows executables
2. copy ./src/test/test_bitcoin.exe to a windows machine
3. download the zcash params ( https://github.com/zcash/zcash/blob/master/zcutil/fetch-params.sh#L11-L16 )
4. move the params files to C:\Users\<windows_username>\AppData\Roaming\ZcashParams
5. Run the tests:

test_bitcoin.exe --run_test=rpc_wallet_tests/rpc_wallet_encrypted_wallet_zkeys
test_bitcoin.exe --run_test=rpc_wallet_tests/rpc_wallet_encrypted_wallet_sapzkeys

Expected behaviour

The tests should pass

Actual behaviour + errors

After running one of these two tests, the runner fails with an error that looks like this:

This application has requested the Runtime to terminate it in an unusual way.
Please contact the application's support team for more information.
terminate called after throwing an instance of 'boost::filesystem::filesystem_error'
  what():  boost::filesystem::remove: The process cannot access the file because it is being used by another process: "C:\Windows\TEMP\test_bitcoin_1540957381_88827"

Also, if test_bitcoin.exe is run with a group of tests (e.g. without the --run_test flag), it halts when it hits this error instead of running the rest of the tests.

The version of Zcash you were using:

2.0.1

Machine specs:

Windows 10