Rename @targetName related tests: alpha -> targetName

Cleanup some leftovers from when `@targetName` was called `@alpha`.

Author: griggt

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -171,7 +171,7 @@ class CompilationTests {
         defaultOptions),
       compileFile("tests/neg-custom-args/i6300.scala", allowDeepSubtypes),
       compileFile("tests/neg-custom-args/infix.scala", defaultOptions.and("-source", "future", "-deprecation", "-Xfatal-warnings")),
-      compileFile("tests/neg-custom-args/missing-alpha.scala", defaultOptions.and("-Yrequire-targetName", "-Xfatal-warnings")),
+      compileFile("tests/neg-custom-args/missing-targetName.scala", defaultOptions.and("-Yrequire-targetName", "-Xfatal-warnings")),
       compileFile("tests/neg-custom-args/wildcards.scala", defaultOptions.and("-source", "future", "-deprecation", "-Xfatal-warnings")),
       compileFile("tests/neg-custom-args/indentRight.scala", defaultOptions.and("-no-indent", "-Xfatal-warnings")),
       compileDir("tests/neg-custom-args/adhoc-extension", defaultOptions.and("-source", "future", "-feature", "-Xfatal-warnings")),