add test for julia install and mtime maintenance (#200)

.github/scripts/common-tests.jl

@@ -0,0 +1,11 @@
+if !occursin("hostedtoolcache", Sys.BINDIR)
+    error("the wrong julia is being used: $(Sys.BINDIR)")
+end
+if VERSION >= v"1.7.0" # pkgdir was introduced here, and before then mtime wasn't a problem so just skip
+    using Pkg
+    src = pkgdir(Pkg, "src", "Pkg.jl")
+    # mtime is when it's compressed, ctime is when the file is extracted
+    if mtime(src) >= ctime(src)
+        error("source mtime ($(mtime(src))) is not earlier than ctime ($(ctime(src)))")
+    end
+end