tests: fix this too opinionated test suite to work on my machine

Before those fixes, running the test suite resulted in the following
errors:
```
running 9 tests
test tests::test_can_tail_file_with_directory ... ok
test tests::test_can_tail_file_with_nonexistent_file ... FAILED
test tests::test_can_tail_file_with_existing_file ... ok
test tests::test_can_tail_file_with_unreadable_file ... ok
test tests::test_run_command_nonexistent_command ... ok
test tests::test_run_command_success ... ok
test tests::test_run_command_with_args ... ok
test tests::test_main_fails_without_fallback_command ... FAILED
test tests::test_run_command_calling_tailor_again ... ok

failures:

---- tests::test_can_tail_file_with_nonexistent_file stdout ----

thread 'tests::test_can_tail_file_with_nonexistent_file' panicked at src/main.rs:114:9:
Should not be able to tail a nonexistent file
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

---- tests::test_main_fails_without_fallback_command stdout ----

thread 'tests::test_main_fails_without_fallback_command' panicked at /home/skia/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5:
Unexpected success
command=`"/home/skia/workspace/ubuntu/tailor/target/debug/tailor" "/tmp/nonexistent_file_12345"`
code=0
stdout=""
stderr=""

failures:
    tests::test_can_tail_file_with_nonexistent_file
    tests::test_main_fails_without_fallback_command

test result: FAILED. 7 passed; 2 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.02s
```

I believe this is because the test suite is too opinionated, and relies
on an arbitrary file not existing.
I happened to use that important file in my personal workflow, hence the
test suite broke.
Thankfully, I'm a nice contributor and was able to use my skills to fix
this. It now passes on my machine. Thank you.

Author: Hyask

src/main.rs

@@ -110,7 +110,10 @@ mod tests {
 
     #[test]
     fn test_can_tail_file_with_nonexistent_file() {
-        let result = can_tail_file("/tmp/nonexistent_file_12345").unwrap();
+        let temp_file = NamedTempFile::new().unwrap();
+        let file_path = temp_file.path().to_str().unwrap();
+        std::fs::remove_file(file_path).unwrap();
+        let result = can_tail_file(file_path).unwrap();
         assert!(!result, "Should not be able to tail a nonexistent file");
     }
 
@@ -212,8 +215,11 @@ mod tests {
 
     #[test]
     fn test_main_fails_without_fallback_command() {
+        let temp_file = NamedTempFile::new().unwrap();
+        let file_path = temp_file.path().to_str().unwrap();
+        std::fs::remove_file(file_path).unwrap();
         let mut cmd = Command::cargo_bin("tailor").unwrap();
-        cmd.arg("/tmp/nonexistent_file_12345")
+        cmd.arg(file_path)
             .assert()
             .failure()
             .stderr(contains("no fallback command"));