fix: prefer ISO date format

[shetzel]

What does this PR do?

Changes test results start time to be ISO format. The JUnit reporter still reports the start time property in the locale format. The formatStartTime date utility also has the same default behavior but now also accepts an additional parameter for a date format (ISO or locale).

What issues does this PR fix or reference?

@W-10226817@
forcedotcom/cli#2220

Functionality Before

When a language format other than en_US was used to get the ISO time string from a locale specific time string an error would be thrown.

Functionality After

No error is thrown. JUnit test start time is still reported in the user locale. Test results JSON have the start time in ISO format.

[WillieRuemmele]

QA Notes

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✅ : LANG=en_CA.UTF-8 ../../oss/plugin-apex/bin/dev apex:run:test --output-dir test -w 10 -> same output as successful run
✅ : --json output
✅ : with changes, without the env var
✅ : --result-format -> would throw an error before printing, so new time format isn't breaking
✅ : no diffs between junit output
✅ : new NUT passes with library linked
✅ : NUT fails when unlinked