Force MySQL Go test to fail on GitHub workflow if the MySQL test database is not available

[roger2hk]

#21

Run go test -v -race ./storage/mysql/... -is_mysql_test_optional=false
  go test -v -race ./storage/mysql/... -is_mysql_test_optional=false
  shell: /usr/bin/bash -e {0}
  env:
    DB_DATABASE: test_tessera
    DB_USER: root
    DB_PASSWORD: root
go: downloading go1.22.5 (linux/amd64)
go: downloading github.com/go-sql-driver/mysql v1.8.1
go: downloading github.com/transparency-dev/merkle v0.0.2
go: downloading k8s.io/klog/v2 v2.130.1
go: downloading github.com/transparency-dev/formats v0.0.0-20240715203801-9ff9b9e3905f
go: downloading golang.org/x/mod v0.19.0
go: downloading github.com/globocom/go-buffer v1.2.2
go: downloading golang.org/x/exp v0.0.0-20220827204233-334a2380cb91
go: downloading golang.org/x/crypto v0.25.0
go: downloading filippo.io/edwards25519 v1.1.0
go: downloading github.com/go-logr/logr v1.4.2
I0801 14:54:00.601791    4259 mysql_test.go:38] Successfully connected to MySQL test database
testing: warning: no tests to run
PASS
ok  	github.com/transparency-dev/trillian-tessera/storage/mysql	1.010s [no tests to run]

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 27.44%. Comparing base (46ec9c2) to head (a616826).
Report is 26 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #107      +/-   ##
==========================================
- Coverage   35.80%   27.44%   -8.37%     
==========================================
  Files          16       19       +3     
  Lines        1363     1771     +408     
==========================================
- Hits          488      486       -2     
- Misses        801     1211     +410     
  Partials       74       74              

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