Fix minor error surfaced by clippy in Rust 1.56

[thanethomson]

Running cargo clippy --all-features --all-targets -- -Dwarnings -Drust-2018-idioms with Rust 1.56 results in:

error: unneeded `return` statement
   --> testgen/src/tester.rs:464:37
    |
464 | ...                   return;
    |                       ^^^^^^^ help: remove `return`
    |
    = note: `-D clippy::needless-return` implied by `-D warnings`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_return

This minor PR fixes that error.

• Referenced an issue explaining the need for the change
• Updated all relevant documentation in docs
• Updated all code comments where relevant
• Wrote tests
• Added entry in .changelog/

[codecov-commenter]

Codecov Report

Merging #1010 (0a30965) into master (7637b2f) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

❗ Current head 0a30965 differs from pull request most recent head 841bea2. Consider uploading reports for the commit 841bea2 to get more accurate results

@@           Coverage Diff            @@
##           master   #1010     +/-   ##
========================================
- Coverage    72.8%   72.7%   -0.2%     
========================================
  Files         206     206             
  Lines       16635   16651     +16     
========================================
- Hits        12126   12109     -17     
- Misses       4509    4542     +33     

Impacted Files	Coverage Δ
testgen/src/tester.rs	72.1% <100.0%> (-0.2%)	⬇️
tendermint/src/abci/transaction.rs	52.2% <0.0%> (-15.0%)	⬇️
config/src/config.rs	63.4% <0.0%> (-1.7%)	⬇️
abci/src/server.rs	10.0% <0.0%> (-0.7%)	⬇️
light-client/src/types.rs	41.6% <0.0%> (-0.2%)	⬇️
testgen/src/vote.rs	85.5% <0.0%> (+0.8%)	⬆️
p2p/src/secret_connection/protocol.rs	69.1% <0.0%> (+1.0%)	⬆️
tendermint/src/block/commit_sig.rs	78.0% <0.0%> (+2.0%)	⬆️
tendermint/src/signature.rs	60.7% <0.0%> (+4.0%)	⬆️

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[thanethomson]

Already merged in #1011