cli: Lookup address by account name, add support for test accounts

[matevz]

Fixes #891.

This PR adds the following to oasis accounts command:

• if address equals account name in the wallet, it returns the address corresponding to the account
• if address starts with test:, it finds the corresponding testing account (Alice, Bob, Charlie etc.) and returns the testing account address
• if --account value starts with test:, it takes the testing account key to sign the transaction
• updates minimal-runtime docs accordingly
• fixes crash when calling oasis accounts withdraw <amount> [to] without to parameter

Example usage:

$ oasis accounts show mywallet
Address: oasis1qqgwegnh3h6p0828npmqed0f243fd9pyhcgmmmt4
Nonce: 0

=== CONSENSUS LAYER (testnet) ===
  Total: 0.0 TEST
  Available: 0.0 TEST

$ oasis accounts show test:bob
Address: oasis1qrydpazemvuwtnp3efm7vmfvg3tde044qg6cxwzx
Nonce: 0

=== CONSENSUS LAYER (testnet) ===
  Total: 0.0 TEST
  Available: 0.0 TEST

$ oasis accounts transfer 1 test:alice --paratime emerald --account test:dave
You are about to sign the following transaction:
{
  "v": 1,
  "call": {
    "method": "accounts.Transfer",
    "body": "omJ0b1UA84957B5s/pe0/gbHiYtSqPrbR4NmYW1vdW50gkgN4Lazp2QAAEA="
  },
  "ai": {
    "si": [
      {
        "address_spec": {
          "signature": {
            "secp256k1eth": "AwF6GNjbybMzhi3XRj5R1oTiMMkO1nAwB7NZAlH1X4BE"
          }
        },
        "nonce": 27
      }
    ],
    "fee": {
      "amount": {
        "Amount": "232100000000000",
        "Denomination": ""
      },
      "gas": 2321
    }
  }
}

Account:  test:dave
Network:  testnet
Paratime: emerald
? Sign this transaction? (y/N)

$ oasis accounts withdraw 1 test:erin --paratime emerald --account test:alice
Error: test account 'erin' (oasis1qqcd0qyda6gtwdrfcqawv3s8cr2kupzw9v967au6) will not be able to sign transactions on consensus layer

$ oasis accounts withdraw 1 test:bob --paratime emerald --account test:alice
You are about to sign the following transaction:
{
  "v": 1,
  "call": {
    "method": "consensus.Withdraw",
    "body": "omJ0b1UAyND0Wds45cwxynfmbSxEVty+tQJmYW1vdW50gkgN4Lazp2QAAEA="
  },
  "ai": {
    "si": [
      {
        "address_spec": {
          "signature": {
            "ed25519": "NcPzNW3YU2T+ugNUtUWtoQnRvbOL9dYSaBfbjHLP1pE="
          }
        },
        "nonce": 13
      }
    ],
    "fee": {
      "amount": {
        "Amount": "1131500000000000",
        "Denomination": ""
      },
      "gas": 11315,
      "consensus_messages": 1
    }
  }
}

Account:  test:alice
Network:  testnet
Paratime: emerald
? Sign this transaction? (y/N)

[codecov]

Codecov Report

Merging #1038 (7cec9c2) into main (a1253bf) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1038      +/-   ##
==========================================
+ Coverage   67.96%   68.00%   +0.03%     
==========================================
  Files         128      128              
  Lines       11017    11030      +13     
==========================================
+ Hits         7488     7501      +13     
  Misses       3497     3497              
  Partials       32       32              

Impacted Files	Coverage Δ
client-sdk/go/testing/testing.go	95.23% <ø> (ø)
client-sdk/go/helpers/address.go	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a1253bf...7cec9c2. Read the comment docs.

[kostko]

Nice, just some minor things.