Fixtures: fix typos (#8028)

Review and correct all typos in the `Fixtures` directory.

### Motivation:

To enhance the overall quality of the project, I reviewed all files in
the `Fixtures` directory and fixed any identified typos.

### Modifications:

A typo was found in a string, and another in a test function name. Both
have been corrected.

### Result:

These changes are backward compatible since they only involve
non-functional elements and do not affect the code execution.

Author: MojtabaHs

Fixtures/Miscellaneous/CheckTestLibraryEnvironmentVariable/Tests/CheckTestLibraryEnvironmentVariableTests/CheckTestLibraryEnvironmentVariableTests.swift

@@ -1,7 +1,7 @@
 import XCTest
 
 final class CheckTestLibraryEnvironmentVariableTests: XCTestCase {
-    func testEnviromentVariable() throws {
+    func testEnvironmentVariable() throws {
         let envvar = ProcessInfo.processInfo.environment["SWIFT_TESTING_ENABLED"]
         XCTAssertEqual(envvar, "0")
     }

Fixtures/Miscellaneous/MissingDependency/Bar/Package.swift

@@ -4,7 +4,7 @@ import PackageDescription
 let package = Package(
     name: "Bar",
     dependencies: [
-        .package(url: "../NonExistantPackage", from: "1.0.0"),
+        .package(url: "../NonExistentPackage", from: "1.0.0"),
     ],
     targets: [
         .target(name: "Bar", path: "./"),