Remove configure_command_msys* tests at least for now

Because they are not really testing the right thing, due to the
distinction between inherited and `.env()`-specified environment.

See #1578.

While for `configure_command_clears_external_config` this slightly
diminishes but largely leaves intact the value of the test, the
`configure_command_msys` and `configure_command_msys_extends` tests
are really verifying almost nothing, and the latter (for this
reason) cannot pass.

A variant of it for testing the preceding fix to append to
arbitrary environment variable values would likewise not be able to
pass. So that is not added at this time.

Because the `configure_command_clears_external_config` test remains
useful, it is not removed.

tests/tools/src/lib.rs

@@ -872,7 +872,6 @@ impl<'a> Drop for Env<'a> {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use std::ffi::OsStr;
 
     #[test]
     fn parse_version() {
@@ -933,44 +932,4 @@ mod tests {
         assert_eq!(lines, Vec::<&str>::new(), "should be no config variables from files");
         assert_eq!(status, 0, "reading the config should succeed");
     }
-
-    #[test]
-    fn configure_command_msys() {
-        let temp = tempfile::TempDir::new().expect("can create temp dir");
-        let mut cmd = std::process::Command::new("not-actually-run");
-
-        let msys_option = configure_command(&mut cmd, &[], temp.path())
-            .get_envs()
-            .find(|(k, _)| *k == OsStr::new("MSYS"))
-            .expect("should customize MSYS variable")
-            .1
-            .expect("should set, not unset, MSYS variable")
-            .to_str()
-            .expect("valid UTF-8")
-            .trim_matches(' ');
-
-        assert_eq!(msys_option, "winsymlinks:nativestrict");
-    }
-
-    #[test]
-    fn configure_command_msys_extends() {
-        let old_msys = r"error_start:C:\gdb.exe";
-        let temp = tempfile::TempDir::new().expect("can create temp dir");
-        let mut cmd = std::process::Command::new("not-actually-run");
-        cmd.env("MSYS", old_msys);
-
-        let msys_options = configure_command(&mut cmd, &[], temp.path())
-            .get_envs()
-            .find(|(k, _)| *k == OsStr::new("MSYS"))
-            .expect("should customize MSYS variable")
-            .1
-            .expect("should set, not unset, MSYS variable")
-            .to_str()
-            .expect("valid UTF-8")
-            .split(' ') // Just spaces, not arbitrary whitespace.
-            .filter(|s| !s.is_empty())
-            .collect::<Vec<_>>();
-
-        assert_eq!(msys_options, vec![old_msys, "winsymlinks:nativestrict"]);
-    }
 }