Implement tests for period file parsing

EconDataLens.Tests/Fixtures/cu.period.empty

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+period	period_abbr	period_name

EconDataLens.Tests/Fixtures/cu.period.sample

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+period	period_abbr	period_name
+M01	JAN	January
+S03	AN AV	Annual Average

EconDataLens.Tests/Services/CpiPeriodParserTests.cs

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+using EconDataLens.Core.Configuration;
+using EconDataLens.Core.Interfaces;
+using EconDataLens.Core.Entities.Cpi;
+using EconDataLens.Services;
+using Microsoft.Extensions.Options;
+
+namespace EconDataLens.Tests.Services;
+
+public class CpiPeriodParserTests
+{
+    private ICpiDataFileParser _parser;
+
+    [SetUp]
+    public void SetUp()
+    {
+        // This isn't actually used in the test, but the parser requires it.
+        var blsOptions = Options.Create(new BlsOptions
+        {
+            Cpi = new CpiOptions
+            {
+                FootnoteFile = "cu.footnote"
+            }
+        });
+
+        // This isn't actually used in the test, but the parser requires it.
+        var downloadOptions = Options.Create(new DownloadOptions
+        {
+            DownloadDirectory = "downloads",
+            DeleteDownloadedFiles = false
+        });
+
+        _parser = new CpiDataFileParser(blsOptions, downloadOptions);
+    }
+
+    [Test]
+    public async Task ParseCpiPeriodAsync_HeaderOnly_YieldsNoResults()
+    {
+        var path = Path.Combine(TestContext.CurrentContext.TestDirectory, "Fixtures", "cu.period.empty");
+        var rows = new List<CpiPeriod>();
+
+        await foreach (var row in _parser.ParseCpiPeriodsAsync(path))
+            rows.Add(row);
+
+        Assert.That(rows, Is.Empty);
+    }
+
+    [Test]
+    public async Task ParseCpiPeriodAsync_FileWithRecords_YieldsResults()
+    {
+        var path = Path.Combine(TestContext.CurrentContext.TestDirectory, "Fixtures", "cu.period.sample");
+        var rows = new List<CpiPeriod>();
+
+        await foreach (var row in _parser.ParseCpiPeriodsAsync(path))
+            rows.Add(row);
+
+        Assert.That(rows, Has.Count.EqualTo(2));
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(rows[0].Period, Is.EqualTo("M01"));
+            Assert.That(rows[0].PeriodAbbreviation, Is.EqualTo("JAN"));
+            Assert.That(rows[0].PeriodName, Is.EqualTo("January"));
+
+            Assert.That(rows[1].Period, Is.EqualTo("S03"));
+            Assert.That(rows[1].PeriodAbbreviation, Is.EqualTo("AN AV"));
+            Assert.That(rows[1].PeriodName, Is.EqualTo("Annual Average"));
+        });
+    }
+
+}