reduce bufreader size from default(8kb) to 1kb

Headers WAY less than 1kb anyway, so this can be improved more?

before

==18725== Total:     429,769,246 bytes in 957,259 blocks
==18725== At t-gmax: 8,058,316 bytes in 3,502 blocks
==18725== At t-end:  3,045,261 bytes in 1,097 blocks
==18725== Reads:     431,872,599 bytes
==18725== Writes:    214,738,653 bytes

after

==49344== Total:     201,418,575 bytes in 957,174 blocks
==49344== At t-gmax: 7,937,250 bytes in 3,310 blocks
==49344== At t-end:  3,035,637 bytes in 1,076 blocks
==49344== Reads:     431,607,448 bytes
==49344== Writes:    210,731,540 bytes

src/tools/compiletest/src/header.rs

@@ -650,7 +650,7 @@ fn iter_header_extra(
 
     let comment = if testfile.extension().is_some_and(|e| e == "rs") { "//" } else { "#" };
 
-    let mut rdr = BufReader::new(rdr);
+    let mut rdr = BufReader::with_capacity(1024, rdr);
     let mut ln = String::new();
     let mut line_number = 0;