Minor correctness cleanups in reporters and helpers

[dalberola]

Low-severity correctness items (agent-reported; [needs repro]).

• configureReporters default case missing return. packages/dredd/lib/configureReporters.js: the default branch constructs a BaseReporter then discards it.
• BaseReporter invalid date. packages/dredd/lib/reporters/BaseReporter.js: new Date(test.start) without a presence guard yields Invalid Date / NaN durations when test.start is missing.
• isURL not case-insensitive. packages/dredd/lib/isURL.ts: /^http(s)?:\/\// rejects HTTP://...; use the i flag.
• reporterCount has no negative guard in the emitStart/emit accounting (TransactionRunner.js); === 0 can be missed if a callback fires more than once.
• Dead duplicate else-if in packages/dredd/lib/reporters/ApiaryReporter.js:71 (both branches assign the same value; flagged by an inline eslint-disable). Already noted locally; fix the logic or drop the branch.

Tasks

• Add the missing return in configureReporters.
• Guard test.start before new Date(...).
• Add i flag to the isURL regex.
• Make reporter-count completion fire exactly once.
• Resolve the ApiaryReporter._get duplicate branch.

[dalberola]

PR #15 implements 4 of 5 items: configureReporters default-case return, isURL case-insensitivity, ApiaryReporter._get dead-branch removal, and emitStart/emitEnd once-guards. Verified against 82 + 172 unit tests, eslint + tsc clean. Deferred: the BaseReporter.test.start NaN-duration guard (abnormal-flow edge case, no repro — best with a small refactor + regression test). Leaving #8 open for that item.