fix flaky test FieldDateWithCustomTemplate

[gpop63]

The test Test_FieldDateWithCustomTemplate sometimes fails due to timing issues.

?       [github.com/elastic/elastic-integration-corpus-generator-tool](http://github.com/elastic/elastic-integration-corpus-generator-tool)    [no test files]
?       [github.com/elastic/elastic-integration-corpus-generator-tool/pkg/genlib/fields](http://github.com/elastic/elastic-integration-corpus-generator-tool/pkg/genlib/fields)  [no test files]
ok      [github.com/elastic/elastic-integration-corpus-generator-tool/cmd](http://github.com/elastic/elastic-integration-corpus-generator-tool/cmd)        0.004s
ok      [github.com/elastic/elastic-integration-corpus-generator-tool/internal/corpus](http://github.com/elastic/elastic-integration-corpus-generator-tool/internal/corpus)    0.003s
ok      [github.com/elastic/elastic-integration-corpus-generator-tool/internal/settings](http://github.com/elastic/elastic-integration-corpus-generator-tool/internal/settings)  0.003s
ok      [github.com/elastic/elastic-integration-corpus-generator-tool/internal/version](http://github.com/elastic/elastic-integration-corpus-generator-tool/internal/version)   0.002s
2024/05/05 23:10:54 rand seed generator initialised with value `4029003210993349982`
2024/05/05 23:10:54 time now generator initialised with value `2024-05-05T20:10:54.918326267Z`
--- FAIL: Test_FieldDateWithCustomTemplate (0.00s)
    generator_with_custom_template_test.go:442: with template: {"alpha":"{{.alpha}}"}
    generator_with_custom_template_test.go:455: Data generated before now, diff: -267ns
FAIL
FAIL    [github.com/elastic/elastic-integration-corpus-generator-tool/pkg/genlib](http://github.com/elastic/elastic-integration-corpus-generator-tool/pkg/genlib) 1.753s
ok      [github.com/elastic/elastic-integration-corpus-generator-tool/pkg/genlib/config](http://github.com/elastic/elastic-integration-corpus-generator-tool/pkg/genlib/config)  0.005s
FAIL

More on this: #144 (comment)