Add test suite for client TxBuilder

[anilcse]

Part of #6213 .

This adds a TxGeneratorTestSuite which tests implementations of client.TxGenerator and client.TxBuilder. It used in this PR to test the legacy StdTxGenerator implementation and will be used in #6385 to test the new protobuf TxGenerator.

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

[codecov]

Codecov Report

Merging #6460 into master will increase coverage by 0.10%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6460      +/-   ##
==========================================
+ Coverage   56.67%   56.78%   +0.10%     
==========================================
  Files         478      478              
  Lines       28745    28745              
==========================================
+ Hits        16291    16322      +31     
+ Misses      11309    11277      -32     
- Partials     1145     1146       +1     

[sahith-narahari]

LGTM