Rename PublicApi to ExecutionContext

This matches the terminology used in the README.

lib/test.js

@@ -30,7 +30,7 @@ Object.keys(assert).forEach(el => {
 	SkipApi.prototype[el] = skipFn;
 });
 
-class PublicApi {
+class ExecutionContext {
 	constructor(test) {
 		this._test = test;
 		this.skip = new SkipApi(test);
@@ -98,13 +98,13 @@ function onAssertionEvent(event) {
 	return ret;
 }
 
-Object.assign(PublicApi.prototype, enhanceAssert({
+Object.assign(ExecutionContext.prototype, enhanceAssert({
 	assert,
 	onSuccess: onAssertionEvent,
 	onError: onAssertionEvent
 }));
 
-Object.defineProperty(PublicApi.prototype, 'context', {enumerable: true});
+Object.defineProperty(ExecutionContext.prototype, 'context', {enumerable: true});
 
 class Test {
 	constructor(title, fn, contextRef, report) {
@@ -176,7 +176,7 @@ class Test {
 		let ret;
 
 		try {
-			ret = this.fn(this._publicApi());
+			ret = this.fn(this._createExecutionContext());
 		} catch (err) {
 			this.threwSync = true;
 
@@ -355,8 +355,8 @@ class Test {
 
 		return this.promise().promise;
 	}
-	_publicApi() {
-		return new PublicApi(this);
+	_createExecutionContext() {
+		return new ExecutionContext(this);
 	}
 }