Add a basic build.zig and main.zig to run tests from multiple modules

.gitignore

@@ -0,0 +1 @@
+.zig-cache

build.zig

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+const std = @import("std");
+
+pub fn build(b: *std.Build) void {
+    const target = b.standardTargetOptions(.{});
+    const optimize = b.standardOptimizeOption(.{});
+
+    const test_step = b.step("test", "Run unit tests");
+
+    const unit_tests = b.addTest(.{
+        .root_module = b.createModule(.{
+            .root_source_file = b.path("./main.zig"),
+            .target = target,
+            .optimize = optimize,
+        }),
+    });
+    const run_unit_tests = b.addRunArtifact(unit_tests);
+    test_step.dependOn(&run_unit_tests.step);
+}

main.zig

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+const std = @import("std");
+
+pub fn main() !void {}
+
+test "main" {
+    _ = @import("./vec3/src/vec3.zig");
+}