Improve formatting in test/parse_benchmark_output.py (#18047)

New:

```
                         Node.js_no_stack_check	Node.js_with_stack_check
                  base64 1.000	0.972
            conditionals 1.000	1.096
                    copy 1.000	1.003
             corrections 1.000	0.982
           corrections64 1.000	1.004
                fannkuch 1.000	1.004
             fasta_float 1.000	1.002
                  havlak 1.000	1.025
                     ifs 1.000	0.962
         matrix_multiply 1.000	1.021
```

Old:

```
 	Node.js_no_stack_check	Node.js_with_stack_check
base64	1.0	0.9715909090909092
conditionals	1.0	1.096219035202086
copy	1.0	1.0028841716658976
corrections	1.0	0.9817155149233665
corrections64	1.0	1.0042210283960094
fannkuch	1.0	1.004495825305074
fasta_float	1.0	1.001837993960877
havlak	1.0	1.0250723240115718
ifs	1.0	0.9619738238415282
matrix_multiply	1.0	1.0212443095599393
```

test/parse_benchmark_output.py

diff --git a/test/parse_benchmark_output.py b/test/parse_benchmark_output.py
old mode 100644
new mode 100755
index 6354fa8..d76c179
--- a/test/parse_benchmark_output.py
+++ b/test/parse_benchmark_output.py
@@ -54,13 +54,20 @@
     for i in range(1, len(line)):
       line[i] = line[i] / base
 
+col0_width = max(len(r[0]) for r in matrix)
+
 # filter results
 result = []
-for line in matrix:
-  if len(line) != len(matrix[0]):
-    print('warning: not enough results, skipping line:', line[0])
+for i, row in enumerate(matrix):
+  if len(row) != len(matrix[0]):
+    print('warning: not enough results, skipping row:', row[0])
   else:
-    result += ['\t'.join([str(x) for x in line])]
+    line = '%*s ' % (col0_width, row[0])
+    if i == 0:
+      line += '\t'.join([str(x) for x in row[1:]])
+    else:
+      line += '\t'.join(['%.3f' % x for x in row[1:]])
+    result.append(line)
 
 # print results
 print()